Fuel Economy Reporting Thread

sleep2much

My 4 litre V8 Jag does 25 mpg on a stretch, and only 17 mpg around town, I knew what I was buying into, if I was getting that from a 1.6 Astra I'd be looking for leaks, terrible.

tahrey

ended up back here lookin for something else, but... hey, why not. just a reply to / ponderance on some points raised here...

1/ Thirsty Astra (and fords) - I'd get that looked at pronto, by a mechanic who knows what they're doing, maybe an independent who specialises in Fords. Could be your exhaust gas sensor (lambda, O2, other names) has given up so the fuel mixture has gone to pot / engine has switched into "limp home" mode (not economic, and produces less power so you hit the throttle more), or your gearbox has gone wrong (i'm suspecting this of a friend's Focus automatic, it doesn't seem like it actually "locks up" on the motorway in top as it should... when he puts a little more/less power on, the revs jump up/down immediately by 2-300rpm but the speed alters more slowly, when it should act like a manual car at these speeds (eg they change in tight unison)... it also gets terrible economy in these situations when it should be only slightly worse than a manual --- modern autos are only typically VERY much worse than manuals around town or in spirited driving, rather than on the motorway, as their drivetrain becomes straight through rather than the soupy mess of thick oil it is at lower speeds/gears).
It may cost a bit to get fixed, either having parts replaced on the box, or replacing it entirely, but it'll be well worth the money. If it turns out to be the sensor, you can probably do it yourself if you're alright with a spanner, or it will not be incredibly expensive for a pro to do it for you. (If it were a Rover, it'd be a different story, as they'd have to take the whole pipe off to get at two different sensors in tricky locations).

Cars will generally give lower economy figures than you get in the books and adverts, which should only be approached as a guide - see if you can find long-term reviews (such as Top Gear's "lifers") instead. The economy tests are done under fairly strict, and not terribly realistic conditions, but the testers have an awful lot of cars to get through in a short amount of time (and money) and so give the best controlled-condition, comparitive, something-akin-to-reality test that they can with limited resources. One of the things is that they're performed on a rolling road, with air resistance only simulated through the wheels rather than having a real effect, and it doesn't seem to be a particularly accurate simulation. Most cars will be a few mpg off, some even end up a little higher (like the petrol Citroen C1, surprising road testers enough to get a quote put on it's billboard ads) --- but it shouldn't be as chronic as people are saying on here with their Astras and Foci. Even mine isn't typically more than 5-6mpg off-target on a bad tank, and that's with an ageing example that's not treated with any particular sympathy, driven in a moderately hilly area and mostly in-town, and is often full of miscellaneous, weighty crap.
Particularly given previous comments about gearing, etc, i'd be inclined to say something's wrong, as these cars seem to have reasonably good auto boxes (what i've seen of them anyway --- and vs, say, a base-model Citroen, or a Volvo, both of which are comparitively sloppy) with good power transmission and tight, economical-pattern changes. Not exactly Mercedes level (rides in diesel-auto Merc taxis make me want to start buying lottery tickets as its like being on a road monorail), but still good and presumably efficient.

2/ Accelerating, low-rpm change ups, etc... Of course changing at lower rpms SHOULD help, but you still have to consider the potentially great expense of engine wear, etc, and it's not just as simple as "lower rpm gives better economy", or we'd all be fitting double-size flywheels, super-long-ratio gearboxes and adjusting the spark timing. Per-horsepower efficiency for a petrol engine isn't that simple - it follows a curved graph, with it being highest somewhere around peak torque output (not necessarily there, but usually close - at least, this is the case for 8v cars I've looked at, might be different for 16v as their peak torque is usually at MUCH higher rpm's) and falling away ever faster as you move away from this point (and similar for diesel, though it seems a slightly flatter curve). Specific efficiency seems to go quite a bit lower at the top end than the bottom, but the engine is still less efficient for it's power production at the lower end of the scale than the 'sweet spot'. However this test is usually only done at full throttle, and doesn't take into account the additional waste in terms of the motor spinning faster and therefore having more friction and reciprocating mass (having to accelerate and decelerate the pistons several times a second is surely a big energy sink) --- whatever it is, there seems to be a beneficial effect in reducing revs slightly below this peak for some reason. Also I've seen experimental evidence to suggest the best efficiency for such an engine is with the throttle fairly wide (e.g. 2/3rds) but not entirely so. Certainly I know that internal resistance also goes up with a narrow opening (probably one of the reasons low-gear cruising is thirsty, as you're using much less of the capability), perhaps it's the switching of the maps or some other thing when you approach full opening. I do know honda / toyota (?) and BMW have made engines where the opening of the actual cylinder valves is regulated to control the amount of fuel/air mix entering (or even just air if it's direct-injection) and have done away with the wasteful, turbulent throttle butterfly altogether.

2a/ As regards the potential cost of engine damage, or even just having to de-coke it after running round all day at 95% throttle and 1200rpm in a car, e.g. like a Clio I once borrowed, that barely makes enough power at that point to maintain forward motion in top gear - this stuff should not be discounted if you're serious about money saving. Like a friend who bought a brand new fiesta, and is about to replace it around the three-year point with another completely new one... and likes the look of the CDTi engine (and did last time) for both the in-gear acceleration and the fuel cost saving - but won't have it because the option price takes it beyond their personal budget... never mind that in three years the option will probably more than pay for itself, or if they're that bothered about money they should keep the same car/get a used CDTi (Focus, for preference, as the low roof of the current Fiesta design makes getting into the back seats challenging - and I'm not even 6ft), as nowadays a five year old fiesta will be just as solid as a new one.... hell, my astra is only starting to show genuine signs of age (stiff passenger window (probably rusted from underuse), occasional harder/more longwinded - but successful - starting on cold/damp days, some fascia rattles from engine harmonics, damaged driver's seat recliner knob... er, that's it) at 10 years old, and it's supposed to be a "flakey vauxhall" not a fairly reliable ford. Forget fuel costs, there's a few thousand pounds of no-brainer money-grubbing for you right away. Doesn't even cost a lot to service.
My mother did similar: brand new fabia (polo too expensive!), and in wanting to control the price, got the worst-efficiency-in-range 1.6L petrol instead of the few-hundred-pounds-more 1.4 or 1.9L TDi. Now she complains about how much petrol costs in her car-reliant job... arrrrgh.
On the other hand, I'm tempted to follow the lead of a friend who recently chopped in his and his girlfriend's ageing cars (Sierra, Fiesta - both very much at the end of their hard-driven lives) for almost direct but much newer replacements, using repossession/lease clearing/etc auctions. Including a sub-£5000 Mondeo CDTi that's only three years old, maybe 50k on the clock, and is still in great condition, well serviced, goes fast ... and turns in over 60mpg on a run when properly looked after, dropping gradually to about 45mpg, which is when you know it's time to get the oil and the air/fuel/oil filters changed. Quite incredible in a car of that size, speed and load capacity (he's a diver, so needs the boot space and higher max weight rating). Plus it's going to last probably another 10 years (if the Sierra's any guide) and he'll likely keep it til then. £5k is a lot of money in my terms, but I can't help but think it's been very well spent.

2b/ Block changing, for efficiency? Don't make me laugh. If you're not revving it too high in one gear, then you're probably labouring it in the next, and probably doing both. The only use it could have is if you're having to pull up to speed quite hard (merging into a busy road, etc) but then drop straight from 2nd or 3rd into top (or 4th, into 6th) once this is acheived, or go from top down to a gear 2 or 3 lower when encountering a big hill and you want to take advantage of engine braking. I can see where the idea's come from - oil crisis era VW's with super long ratio 4-speed boxes where you were more or less block changing from (long-ish) 3rd to (short-ish) 5th, with no 4th gear... but those were products of convenience and manufacturing realism rather than any scientific advantage over a 5 or 6 gear box with a similar overall range (other than being a few, sparing kilograms lighter). IE they had no suitably adaptable 5-speed casing or common internals that could be used for what seemed to be hastily churned out alternative models (particularly the early polos and maybe golfs, which had narrower engine bays/subframes - even the first GTi's/Sciroccos/Polo GTs were 4-speed), so they jammed in what alternative collection of 4 cog pairs that could be found in the parts bin to achieve the desired effect. There's reason I would like to try 6-speed in my own car, besides getting a higher motorway gear; I think having a lower 1st and smaller gaps between the quite wide-set lower speed gears could save a few mpg here and there by keeping the engine in it's efficient range for more of the time, particularly around upper 1st thru lower 3rd gear. There's no real penalty to actually changing gear so long as you do it in a reasonable amount of time - the throttle's closed and the revs are falling (usually) above the over-run cut-off point, so there's not a great excess of fuel going to be lost in that split-second.

3/ Regarding coasting vs staying in gear...... if you ever consider coasting with the clutch down on a hill or in neutral, and it's possible for you to do it safely (headlights/brake lights still work, guages still work - or with only minor interruptions for either - and steering doesn't lock or become excessively heavy), you really should kill the engine at the same time. The speed you lose through engine braking, or the small gasp of fuel to maintain the speed you want, will more than likely be well compensated by that used in order to keep the motor ticking over independently of the car's motion. And if you can make it to the bottom of the hill in gear at the speed you want (hint: if all else fails and you're not in top, change up), just do that - don't turn off the engine (unless it's clearly turning below the overrun fuel cut-off trigger speed... if systems haven't got much better you can usually clock roughly what RPM this is if you're observant by subtle changes in the car's motion, engine noise, etc as it passes the point either slowing down, or accelerating via gravity downhill) and have it run along safely, legally and efficiently as-is.
Remember, idling DOES USE FUEL. And it's not terribly efficient given fuel used vs presumed power made (as it's running very slow, with a very restricted throttle), though it is typically set up to use the least possible fuel that it can without the engine actually stalling out. When you put a load on it, typically the engine governor/ECU will detect it slowing, and in response further extend the little solenoid-actuated stick that presses on the throttle dial (for want of a better descriptive word - the rotating doodad that translates pedal position, via cable, to the actual butterfly valve) and inject a little more fuel, to produce a touch more power and prevent the whole thing coming to an embarrasing halt... the same, pretty much, as you pressing very gently on the pedal, and the effect is more so when you do it in higher gears, to the point where it can become quite juddery and momentarily rev the motor up fairly hard when you hit the clutch, to the point where I don't much do this above 2nd, sometimes 3rd gear any more. Yes you can take your foot off the accelerator and let the car slow down until it trots along at a "minimal" speed determined where the balance between reduction from no-load idle and wider throttle opening is achieved, but it shouldn't be used for anything other than very low speeds in traffic queues (where revving up *would* be silly) --- don't be under the impression that doing it at 25 in top will be particularly efficient, or good for the engine, because it'll be neither, and it WILL use fuel. What Jeremy Clarkson was on about was over-run cutoff, which cuts the fuel supply to the carb/injectors above a certain RPM when the throttle is fully closed, to both improve engine braking and save petrol... takes a second or two to kick in (as on all but direct-injection cars there's a short time lag, caused by the physical distance between injector/carb and cylinder, when you come back on the power even though the supply restarts immediately - which could be annoying or even dangerous if it occurs when you're just backing off for a moment around town or on a country lane), and turns off below a certain rpm (1200~1800, I've found, depending on type of engine, coolant temperature, etc) to protect against stalling, rpm 'hunting' (another reason for the delay) and violent jerking in low gears. So it's useful both to save petrol, and your brake pads, when slowing down coming to a junction or downhill, but you still have to change down a gear to get the greatest benefit, and it ceases to have any effect at all one you drop below a jogging pace.
I suspect it may have been more advanced in the Audi A8 he did the economy test in - cutting fuel until the very last moment and using the last vestiges of the car's momentum to keep it turning down to idle speed, particularly as it's a direct-injection diesel, so if it were a manual you really could leave it in gear almost until you stopped - but this isn't normal... even in mine where it seems to alter the pattern when it senses i've reached the reserve level (<5L remaining... a few litres after the needle hits rock bottom...), removing the time delay and lowering the rpm limit, it still cuts the injectors back in well before you reach idle. You're better once it hits that point in, say, 2nd gear, just using the normal brake and declutching, as it won't be any more or less efficient, will hardly make an impression on your pads, and is just safer and more responsible, plus you don't fool yourself. In fact it might be more efficient, as if you brake at that point where the injectors start back up, you'll be fighting the engine to stop the car, and it'll be fighting you in order to avoid stalling.

3a/ By the way, it's not as simple as "the accelerator makes the fuel go in". There's some truth to that, but you're not operating some kind of pump or even a tap with your foot. The carburettor, and nowadays the injectors, tend to mix fuel with the incoming air in a fairly fixed ratio, typically around 14:1 in free running, which is approximately best for "complete" combustion of petrol... altering the carburettor's choke, or how fast the injectors run (normally under ECU control, in response to temperature, exhaust O2-content, and throttle-movement sensors, following it's ROM mapping) can alter this, sometimes to a "leaner" mixture (less fuel, better economy, though more nitrogen dioxide produced) but usually "richer" (more fuel), which can provide more power (to a certain extent) and make for cooler/smoother/more reliable running (in the case of highly tuned engines prone to heat damage, or any normal ones which are difficult to start from cold without this) at the expense of economy and emissions.
So that's one complication already, before we get on to the alteration of the airflow due to engine speed, throttle setting, and the effects of air temperature (and use of alternative intake paths to change this), air rams and turbos, throttle lag, spark timing, etc

Which is why it's easier to try and reduce it to terms of what you actually do when you're driving, which boil down to how hard you push some pedals, how fast you like to travel, and how you move a stick around, as most of the stuff above can't be individually influenced, but responds in complex but measurable ways to your inputs on these simplified controls. (Which have reduced over the years - we lost the choke control with the coming of fuel injection, and the spark timing and manual warm air diverts, etc etc, much earlier with the coming of more easily automated / low tech parts such as centrifugal and vacuum-advanced distributors, or intake heaters with automated relays)

3b/ Performance ROMs / altered ECUs CAN make a difference. I and some other reformed polonauts are living proof. I did put a chip in mine, as its engine's one nod to modernity was electronic fuel injection & ignition (the rest being hacked-together relics from VW's earliest water cooled engines). The ECU wasn't the greatest either, quite a primitive Motorola unit, with a ROM map that seemed to have only one particular bit of thought put into it - a slight power notch/flat spot that helped it comply with (at the time) updated Euro emissions regulations. As well as making power-upgrade ROMs for the more powerful engines in the range, a messageboard hobbyist was also turning out chips with improved maps for everything he could get hold of to test, quite cheaply (say, 1/10th the price or less of those advertised in the back of Top Gear? They're VERY cheap to make, probably less than £10 each from scratch for 100, once you've figured out how to program them and suitable figures to use). Mine was sold honestly as not really offering a great power improvement (maybe 1 or 2hp, woo) by enriching the mixture a bit, as there wasn't much more the engine had to give without serious upgrades to the air path and cams (totally not worth it on a 1.0, when 1.3s were readily available), but it did smooth out these flat spots, and lean the mixture out a little more at constant speeds as it was more than capable of coping with it. And improve these it did - no more embarrasing hesitations when trying to make a fast right turn-off uphill and slightly fluffing the throttle, maybe a consistant 1 or 2mpg improvement in general use. Ultimately worth the minimal asking price for a similarly sparing but quantifiable improvement.

However, we still reserved the right to laugh at those who paid £50+ for a "super economy" or "super power upgrade chip" on eBay that proved to be a resistor or capacitor that plugged in between your lambda sensor and ECU (parts - 50p from maplins), tricking it into thinking the engine was running either richer or leaner than it actually was, and altering the mixture accordingly..... with all the classical running and maintenance problems that would come of a carburetted car with a poorly adjusted choke, and actually a very poor improvement, if not a reduction in the desired statistic.

4/ Yes, super impressive economy is available from driving really slowly (or just with the trucks), and doing odd things like Pulse & Glide (accelerate efficiently up to a certain, low speed at the top of the torque band in e.g. 4th gear, kill the engine and coast down to the bottom of said band, then repeat), but we have to realise we're living in the real world, particularly one with other people, with work committments, and with clashing events that can't wait for everyone to arrive. Often you have to travel at a reasonable higher speed to get places in a sensible timescale if you're not just unemployed/studying/retired and effectively pratting about without too much of a care for when you have to set off in order to get somewhere, or even when you arrive. I've had a shared ride between Luton and Birmingham for a group trip this weekend, with our driver ultimately having come from Anglesey making a number of stops along the A55, M6 and M1 to pick up and drop off. At 70-80mph, including congestion, getting lost in unfamiliar towns, and stops at services, it took him nearly 7 hours one way, and over 4 1/2 the other. Doing 55 would have seriously cut the fuel costs (we paid in something like £15 each, plus more for parking) but also make us miss the flight out, and left him with no nap-time between getting home and going out to work. Doing 90-ish would have meant we didn't have to run though the terminal, and left him feeling better this morning (it was a long trip, I calculated the times mentally out of boredom - it made nearly an hour's saving), but he honestly couldn't have afforded the fuel (it's the same thirsty Focus auto), would have had to be very aggressive to slice through the friday traffic, and been easy pickings for the police on the way back. Hypermiling it with a mix of slow cruising and pulse & glide for an average somewhere between 30-40mph (the average speeds people post whilst being all smug about their huge economy are usually VERY low, even when they're taking very long trips) would have probably halved the fuel use, but would have involved his setting off before sunrise and being back late for work - and if another driver or a tree hadn't killed him first, we would have done so. So don't make out that going faster doesn't make any useful difference to journey times, because that's such a bloody lie. Like most things in life that involve time and money, it's a compromise, and you make your best one according to the situation. I've done the entire leg of that journey between his place and mine behind trucks before, as I had literally nothing else to do that day other than transport a huge amount of gear (so the train wasn't an option), and I've done it with an average speed approaching three figures once time spent crawling in traffic is discounted, as the delays were threatening to make me late for an important appointment, and the greatly increased fuel spend and slightly increased risk of arrest or mishap were balanced against this.
Really, if you're THAT committed about saving money and fuel whilst travelling, and don't have a great deal of gear to move between two obscure locations, you're better off looking for a good deal on a public transport ticket, or getting a pushbike or low-performance motorbike, rather than causing risk and frustration to yourself and others.

I so wish I had a car with a trip computer, or that was compatible with a ScanGuage, so that I could properly log these things (it's typically incompatible with SPi cars, and particularly the ones I've had) and confirm for myself the truth of various ideas, rather than having to stick in the theoretical realm and relying on others' research and reports. I can retrofit an official vauxhall one, should I be able to find it, but apparently it's a dash-off rewiring job rather than just swapping out the display panel plugging a new one into a multi-way connector.... so sod that.

barnpot_2

I have a 1 year old 1 ltre toyota yaris. I recently changed my job which meant commuting 11 miles on the motorway, I discovered that if I kept my speed around 60mph, I got and extra 50 miles out of the tank, than driving at higher speeds through country etc.

Gallovidian

jamesOMGthatsbig wrote: »

my way of increasing/saving diesel is run 50-50 mix of diesel and veggie oil (56p per litre) and slipstreaming HGVs. I know it's naughty and potentially hazardous .

Its perfectly legal to use vegetable oil in your car as a fuel, as long as you use less than 2500litres per year. Simply write to the inland revenue and they will issue you with a 'de minimis' stating that you are not expected to pay duty. The limit of 2500l is per household. Some diesel engines are not compatible with vegetable oil in any quantity, especially modern direct injection diesels. There are loads of websites which discuss this in great detail and can advise on engine compatibility, one such is 'vegetableoildiesel.co.uk'.

It is a very good way to save money if you can do it and I personally can see no ethical objection.:T

Gallovidian

It is also worth checking to see if your vehicle has a viscous coupled cooling fan. If so and you don't do a lot of towing, or live in a hot climate, have it removed. A thermostatically controlled 'Kenlowe' type electric fan will do the job and use a lot less energy to do it. Fuel savings of about 5% are reported from removing the viscous fan.;)

Paulo303-2007

I found that if I change gear at 2000rpm instead of 2500 or 3000 I get around 6mpg more. Also BP Ultimate enabled me to get from cornwall to winchester for just £10!!

Enm3nh

I drive a 54 plate Renault Clio 1.5 dCi and the fuel consumption is amazing. I average around 62 mpg. If I am not driving to conserve fuel this can bea s low as 55 mpg (still pretty good). The best fuel economy I have got over an 80 round trip was 87 mpg! Shows there is money to be saved.

DKLS

I have a Subaru Forester S Turbo, with a few discreet modifications, On a run I get 28-29MPG, On daily commute about 23MPG, If I put my toe down and enjoy the car around 18MPG.

I would replace with a diesel but cant afford the only diesel I would actually be willing to own

tomstickland

I did 780 miles in each direction on a trip to the Alps from Gloucester. Steady 80-85 indicated the whole way on motorways, plus approx 200 miles of A roads in France. Heavy rain for the entire return journey. Achieved 35mpg. 1991 BMW 318iS (1.8 16v) with 210,000 miles on it.

It is also worth checking to see if your vehicle has a viscous coupled cooling fan.
.... Fuel savings of about 5% are reported from removing the viscous fan.

Sounds interesting, since I do have a viscous fan.

jamfer

FUEL SAVING TIPS.

The coolest time of day is around 6.00 a.m., so best to fill up around that time if practical - though don't drive miles to do it! Warmest time about 2.00 p.m..

Driving with a full tank obviously adds weight.

A light right foot is good: tearing around costs more in fuel and brake pads! People who speed up to red lights and jam on their brakes not sane! (OK - I was a boy racer once!)

I have used LPG on variouos cars for some years now. I currently pay 52p with a contract with Countrywide Stores (only available in the Midlands at presenyt) and pay monthly, thus saving a few more pennies. Morrisons LPG is cheap when you can find it. LPG conversion not cheap and tanks can take a lot of space, and can also mean slightly more frequent servicing, but what savings! Fuels consumption better if anything.

I also use a magnetic fuel enhancer. This has improved efficiency dramatically on some vehicles I have used it on, not at all on others. I can give details if anyone wants - svs@phonecoop.coop or info@singlesfestivals.co.uk.

Simon

MSE_Martin wrote: »

This thread links on the back of the 'Cheapest Petrol & Diesel: Drive down the cost' article

Please read the article first, as this thread refers to it

If you're resolving to save fuel by driving more carefully, I'd love to know how well it works. There's no doubt that following the tips in the article can get you more miles per gallon, but exactly how many more can you expect? Please post any findings below, and I'll include them in the next rewrite.

Suggested Method

Work out how many MPG you are currently doing on your standard driving/commute.

Then for the whole next tank of petrol sort your tires, roof-rack etc out and drive more smoothly and see what difference it makes.

Martin.