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[MSE Lawrence]

This is the discussion to link on the back of Martin's 'Petrol efficiency experiment; an increase of 20%' blog. Please read the blog first, as this discussion follows it.

Read Martin's 'Petrol efficiency experiment: an increase of 20%' Blog.

For full info on cutting the cost of driving, read the
Cheapest Petrol and Diesel article.

Click reply to discuss below.

[llandaff]

Good trial, Martin, and no doubt you saved petrol, but how much? The onboard computer in cars is not reliable and especially so when you are measuring each trip separately. A better way but a little bit more work is what I have been using over many years.
First start with a full tank; this is not difficult as the cut out on most pumps uses the pressure from your tank. Record mileage at start and again when you next fill up. Using the recorded difference in mileage and the amount of petrol needed to refill the tank it is easy to calculate the mpg. Every time I do this it differs from the computer average. Having tried this on many cars over the years I have always found my method reliable and the computer rarely agrees.
Another check on the onboard computer is to wait until it flags up the warning: only X miles of petrol left. Try driving those X miles to see what happens (you may need to carry a spare can of petrol!) .

[fiona10]

hi martin, i too am currently doing the same experiment. i drive a renault clio 1.2 and it is fairly ecconomical. i reset the fuel consumtion on the dashboard computer every day and watch it as im driving around and was amazed when on the motorway that if i cut my speed by 10 mph the fuel consumption changed drastically. and like you said it dosnt take you any longer to get to your destination either. im also currently shopping around for the cheapest petrol at the moment as my car is by far my biggest monthly outgoing.

[melancholly]

i only just found the setting on my car for telling me the average MPG - it's a little car and my current average is 39.4..... and i think it should be better than that. i can't seem to reset it by trip yet (i will get out the manual this weekend though after reading the blog). i'm definitely watching the way i drive though - anything that saves petrol costs at the moment is a great idea!

[wyze]

My trip computer is broken but I use the one on this website which remembers past results and is handy for comparison. http://www.torquecars.com/tools/uk-mpg-calculator.php

[anewman]

Originally Posted by llandaff

First start with a full tank; this is not difficult as the cut out on most pumps uses the pressure from your tank.

I think different pumps use different cut off levels and it also depends on other things like temperature of the tank (with heat liquid expands) and probably also how level the car is as this will affect the level the petrol comes up to at the bit you fill up at. Anyone with an older car that has a petrol guage designed without a stabiliser will see the effect of this when going around corners (suddenly more petrol, then less petrol, then more).

While not scientifically precise I think it is the best approach. I would assume the onboard computers base the calculation on how much petrol flows through the injectors so should be a good estimate, if not precise and perfect.

[indierocker85]

Was sat at Shell today and staring at the price and thinking, How much longer will I keep my car? As I have a moderate income but the price of petrol os ridiculous, I currently drive a 1.3 fiesta, and I have just read the article on petrol. Sainsburys is always the cheapest and closest for me, plus i can collect nectar points as well.

I am planning to try the following, filling up my tank and driving as normal and seeing how many miles I get from a full tank of 40 litres.When this is done and I work out my average MPG, I am going to completely declutter the car, check the tyre pressures are spot on and drive as martin as suggested to see what the difference is between the two MPG averages. I am sure the results will shock me

It's interesting to think, what could we as drivers and tax payers realistically do to try and bring down the cost of petrol? Protests? Boycotts?

Also where I live the most expensive station is at 112.9 for Unleaded (BP) and the cheapest is 103.9 (sainsburys), the two stations are literally about 200 yards apart yet bizarely people still go to the BP garage effectively wasting 9p a litre.

If I can't save by trying the above, I'm afraid I'll ditch the car and adapt the money saving train commute again

[anewman]

Originally Posted by indierocker85

I am planning to try the following, filling up my tank and driving as normal and seeing how many miles I get from a full tank of 40 litres.When this is done and I work out my average MPG, I am going to completely declutter the car, check the tyre pressures are spot on and drive as martin as suggested to see what the difference is between the two MPG averages. I am sure the results will shock me

Just to say maintaining tyre pressures is a safety issue and if found to have under-inflated tyres you can be fined and get points from the police. Under-inflated tyres will also wear differently and be more likely to need replacement quickly. Also, forget the filling station air. It's usually inaccurate and when you've driven there your tyres are warm anyway which affects the reading.

I believe biggest improvements can be gained from maintaining/servicing the car properly and as often as needed, and also driving at the speed quoted by your car manufacturer as having the best MPG, usually about 56mph, instead of 70 or 80mph on the motorway. The faster you drive, the less fuel efficient your car will be over the distance you travel. Sitting at 56 you can also stick in the inside lane unless other people really are travelling slower, and you therefore are at less risk of accidents too. Sitting behind the lorries can also benefit fuel efficiency in terms of aerodynamics too

[kaya]

you cannnot accuratley judge fuel efficiency by volume guys and gals, sorry, petrol increases and decreases in volume according to its temperature, the only way to accurately judge fuel economy would be to weigh the fuel in the tank before and after the test period, trip computers in cars arent too good either, putting a different set of wheels/tyres on your car can make a huge difference to the reading also as your changing the outer circumference of the wheels

[caretakerPete]

When I drive I aim to not have to use the brakes, I find this helps greatly, you are always consious of your speed because you have to think ahead, you tend not to drive too close to the car in front, you are constantly aware of the need to slow down at junctions etc and you save on wear to the braking system as well as becomming more confident and calm. I believe if more people adopted this form of driving there would be less problems on the roads The best way to drive is to be relaxed and calm and NOT to go racing around, every time you accelerate heavy you are pouring fuel into the engine, if you suddenly need to stop you strip material off your brakes....
I drive a Ford Scorpio 2.3 and used to spend £50 a week on petrol. I had it converted to LPG and now spend about £25 a fortnight for the same mileage, I don't even bother to monitor the mileage now.
When I first had it converted I went fro HULL to BRISTOL for £18.50... can't be bad...

[charlieheard]

When I started doing a long commute, I bought a Nissan Micra diesel for it's fuel economy. At first I used to commute up and down the A1, trying to make good speed, and I was getting almost 50mpg on the commute. But I soon realised that the congestion was such that it wasn't worth trying too hard, so I settled in to a more relaxed approach. Cars that were desparate to overtake me would still be in sight 30 miles later, so they'd saved themselves 30 seconds by hurrying! My consumption went up to nearly 60mpg. All these readings were taken via the onboard computer, so they're not absolutely accurate, but they were readily repeatable so give an excellent indication.

I decided to try a similar approach on longer business journeys after a "slow" journey. Travelling at normal motorway speeds and keeping up with the traffic, I'd get around 50-55 mpg on a long journey. Then I had a trip where I seemed to follow a succession of wide loads, and my consumption went down (?up) to 78mpg! On the way back, I tried to repeat the approach, and stuck religiously to a 70mph maximum and accelerated more steadily. I got better at this (while still making decent journey times) and managed to get up to almost 80mpg. I often managed over 80mpg average for most of the journey, but it always seemd to drop to 79,9mpg by the end My average speed was only 1 or 2 mph less than previously (3%-ish), but my consumption improved by 40%, so I didn't have to stop so often for fuel, which actually improved my journey times.

I remember an article in CAR magazine where they took a Porsche 911 Turbo across Australia on a derestricted road to see how fast they could manage. Despite travelling at 3-figure speeds, they found that they had to refuel so often that a regular car travelling at 70-odd was keeping pace with them. Hare & Tortoise

[charlieheard]

Originally Posted by caretakerPete

When I drive I aim to not have to use the brakes, I find this helps greatly, you are always consious of your speed because you have to think ahead, you tend not to drive too close to the car in front, you are constantly aware of the need to slow down at junctions etc and you save on wear to the braking system as well as becomming more confident and calm.

I agree wholeheartedly with being aware of your surroundings and avoiding using the brakes: I try to do the same. Have you seen those people on the motorway whose whole journey seems to consist of accelerating close to the car in front and then braking hard while I haven't touched mine for miles. Clarkson said something similar on Top Gear last year.

A quick counterpoint though: brake lights are a warning to cars behind, so using your brakes to illuminate the lights is useful even when you don't actually need to brake yourself. Also repeated engine braking can cause wear to the engine, so should be used moderately (as I'm sure you do) instead of as a replacement for brakes. My driving instructor said, "Which is cheaper to replace: brake pads & discs or a whole engine?"

[kennyboy66]

Originally Posted by kaya

you cannnot accuratley judge fuel efficiency by volume guys and gals, sorry, petrol increases and decreases in volume according to its temperature, the only way to accurately judge fuel economy would be to weigh the fuel in the tank before and after the test period, trip computers in cars arent too good either, putting a different set of wheels/tyres on your car can make a huge difference to the reading also as your changing the outer circumference of the wheels

Providing you are using normal retail outlets that store fuel underground, which reduces temperature variation, and fill up at the same time each day and at the same time of year you would struggle to get a difference of 0.25% in mass of fuel delivered due to temperature related volume changes.

Probably accurate enough for the purpose of this experiment.

[JontyII]

[quote=anewman;7810493]
Easter is not to celebrate his death but his rising from death.
He died on Good Friday not Easter Sunday (when we have the Easter eggs)!

Re. the petrol saving discussion. I always drive taking my foot off the accelerator when approaching roundabouts etc. and am constantly amazed at the number of cars bombing past me and then slamming on their brakes at the last minute. Some also brake at any changes ahead such as a slight curve in the road, cars way in front of them and even traffic coming in the other direction. I believe they think that they either have to have their foot on the accelerator or brake, such a waste of petrol!

[redux]

Errrm, a slight mathematical difference about what Martin said:

Going 20% further on a tankful means the rate of consumption, and hence the cost of a fixed length trip or of all fuel bought, drops by 16.7% not 20%

To make the point clearer by stretching things, going 100% further on a tankful is a 50% cost saving

Many countries define fuel consumption in units that are reciprocal to the way we do, e.g. in litres per 100 km, and maybe that's why

[MushyPeas]

Interseting post as I've been thinking a lot about fuel consumption. I travel a lot between SWest and London. I noticed that if I took the motorway my consumption increased (driving around 70 mph) whereas on the 'old road' up it was averaging 60mph so was more fuel efficient.

Also I purchased a tomtom last year. I keep experimenting with the 'shortage' route versus the 'fastest' route. Sometimes the time different isn't much but I save around 5 miles in distance. Though in the country you do end up going down some little roads! Has anyone else tried this method to save mileage?

My next plan is to calculate how much it costs me to drive a mile, will get around to that soon

[Skeksis]

I have 'experimented' this week by driving more sympathetically and carefully than I normally would and although I don't have a fuel consumption readout on my car, I have found that I haven't used as much fuel as I normally would in a normal working week!

I fill up at a BP garage which luckily has to price match with an Asda station over the road, otherwise it would be out of business!

[redpete]

Originally Posted by anewman

and also driving at the speed quoted by your car manufacturer as having the best MPG, usually about 56mph, instead of 70 or 80mph on the motorway.

56mph is often quoted because this is a standard used to give a common comparison point, it is not the speed at which cars exhibit the best mpg.

[PhoneGuru]

For info, this is also being discussed over on the Advanced Driving UK site at
http://www.advanced-driving.co.uk/bb...?p=29301#29301

Some interesting tips on fuel economy http://www.advanced-driving.co.uk/dr...diet-for-2008/

You might also find the fuel economy spreadsheet useful to calculate your own fuel ecomony
http://www.advanced-driving.co.uk/public/fuelecon.xls

[charlieheard]

Originally Posted by MushyPeas

Interseting post as I've been thinking a lot about fuel consumption. I travel a lot between SWest and London. I noticed that if I took the motorway my consumption increased (driving around 70 mph) whereas on the 'old road' up it was averaging 60mph so was more fuel efficient.

Also I purchased a tomtom last year. I keep experimenting with the 'shortage' route versus the 'fastest' route. Sometimes the time different isn't much but I save around 5 miles in distance. Though in the country you do end up going down some little roads! Has anyone else tried this method to save mileage?

My next plan is to calculate how much it costs me to drive a mile, will get around to that soon

I'm very surprised that your consumption is higher on the motorway unless you're travelling significantly faster than 70mph. One thing that really kills fuel consumption (apart from high speed) is changing your speed. Slowing down for corners and junctions and speed limits around towns all mean that the car using energy to return to the cruising speed. So as well as being slower, non-motorways usually mean poorer consumption. Think of it like going the shortest route on a walk. You lose loads of time and energy climbing hills and descending rather than taking the smoother, longer path.

It also means that the shortest route is seldom the most economical, as it usually involves lots of junctions and minor roads. (Unless you're travelling on a bicycle, of course, where the cruising speed isn't very high, so distance is more important than cruising. )