Fuel Economy Reporting Thread

Rave

Conor wrote: »

Which is right. Accelerating at Wide Open Throttle is the most economical way to accelerate.

Yes, but only in a car which actually has a throttle. In a diesel I doubt it is the best way; I think some direct injection petrol cars (like Mitsubishis) don't have throttles either.

Why the hell would you do this? Modern fuel injection engines and diesels since the dawn of time shut off fuel completely on engine overrun - i.e when you take your foot off the accelerator as you slow. You'd only gain if you had an engine with a carburettor which counts out any petrol engined vehicle made after 1992. All you are doing is breaking the law and not having full control of the vehicle for no reason whatsoever.

Irrespective of whether your foot is on the throttle or not, you're still getting engine braking. In every car I've ever driven, petrol or diesel, the engine braking effect is significant; if you're not in a situation when you need to brake anyway (e.g going down a hill on a motorway) then you might as well switch the engine off as far as I'm concerned. I personally don't think it's dangerous as I leave the engine in gear and can restart it simply by releasing the clutch, and the police would never be able to prove it so whether it's illegal or not is a moot point.

Rubbish. On the economical driving course I did, a graph of speed vs wind resistance showed a fairly constant resistance until you got to 55MPH where the resistance then increased exponentially.

That chart was clearly wrong, then. Wind resistance definitely does increase exponentially with speed, starting from a standstill (unless you've got a head or tailwind, in which case the graph will be skewed).

shimmyshack wrote:

wind resistance (the force you expend fuel to overcome) increases with the square of speed,
so if you go at a steady 50 instead of 100 on the motorway, you use 4 times less fuel (2 squared)

No, you use half as much fuel. At 100mph there's four times as much wind resistance, so you engine (give or take, as there are other contributing factors) is producing four times as much power and using fuel at four times the rate- HOWEVER you are also travelling twice as fast. So, in any given period of time you're using four times the fuel but travelling twice the distance, so your fuel consumption in MPG terms is doubled.

Driving at 55mph instead of 70mph halves your fuel bill,

Not on any car I've ever driven it doesn't, it reduces your bill by 35-40% which is what the physics tells you to expect;).

klint

Rave wrote: »

Yes, but only in a car which actually has a throttle. In a diesel I doubt it is the best way; I think some direct injection petrol cars (like Mitsubishis) don't have throttles either.

I don't think it makes any difference whether or not it has a throttle. When your accelerator is down on the floor, you are using most of the fuel to accelerate. When it's only half-way down, you are using just some of the fuel to accelerate while the rest is being wasted on counteracting the effects of engine braking.

Rave wrote: »

...switch the engine off as far as I'm concerned. I personally don't think it's dangerous as I leave the engine in gear and can restart it simply by releasing the clutch,

Now be careful here, this may be ok in older cars without catalytic converters, but in newer cars you can damage the catalytic converter if you do that. My Mazda's driver's manual has a warning about this. It says you should always start the engine using the starter motor, never bump start it.

Rave wrote: »

...and the police would never be able to prove it so whether it's illegal or not is a moot point.

Why do so many people think it's illegal to coast? Just because the Highway Code says you shouldn't do it, it doesn't mean it's illegal. Which section of the Road Traffic Act says it's illegal?

Rave wrote: »

That chart was clearly wrong, then. Wind resistance definitely does increase exponentially with speed,

No, wind resistance increases with the square of the speed. That is not exponential. An exponential increase would be a much bigger increase at higher speeds. Compare the graphs of y=x^2 with y=k^x.

Rave wrote: »

No, you use half as much fuel. At 100mph there's four times as much wind resistance, so you engine (give or take, as there are other contributing factors) is producing four times as much power and using fuel at four times the rate- HOWEVER you are also travelling twice as fast. So, in any given period of time you're using four times the fuel but travelling twice the distance, so your fuel consumption in MPG terms is doubled.

Correct. But remember that's a bit of a simplification, because there is much more resistance than just air resistance. Fortunately, many of the other types of resistance also increase with the square of the speed, so the simplification still works (approximately.)

Andy_Hamilton

I drive a VW Bora 115TDI 6 speed and I think the trip computer should be fitted as standard to every car. If people know what they are using they can instantly change there ways (if they want to).

I currently drive 100miles per day and aim for 52mpg as a minimum.
Driving hard I have had it as low as 42mpg and driving like miss daisy I have acheived over 72mpg (average over 700 miles)

I have learned a couple of thing from experience.
Never coast as the engine braking is minimal compared to the engine keeping itself ticking over with your fuel.
Driving a VW TDI it's best to change around 2000 rpm for fuel economy but they soot up the turbo and if you don't clear the exhaust once in a while your turbo will simply stop working.
Driving upto 2400rpm gived the turbo a good workout and keeps the soot down.

colin79666

I've recently been experimenting with different fuels in my 1.2 Grande Punto.
I'm going about 50 miles a day along country roads.
With BP unleaded (95.9p/litre) I get over 44mpg according to the trip.
With Tesco unleaded (93.9p/litre) I only get 40mpg.
I haven't done the sums but it looks like Tesco fuel in my circumstances is not cheapest in the long run if I'm filling a typical 3/4 tank.

powermac

I am getting between 51 and 56mpg from my 2005 SEAT Leon FR 1.9TDI 150, which is more than I got from my Prius lease car before it. On top of that, the Leon is much more fun (not to mention quicker and smoother) and on the whole seems cheaper to run. Interestingly that is when running on standard Shell or Texaco Diesel (we don't get the fancy Shell Diesel Extra or V-Power Diesel here in Dorset!) both the economy and refinement drop noticeably when using fuel from Morissons or Tesco.

Davyk155ed_2

ajuniper wrote: »

I used to drive over 60 motorway miles a day and therefore could get a good repeatable economy reading from my car (which calculates it per journey and per tank).

I found that switching from Asda diesel to normal Shell diesel I got about a 5% increase in economy. This was repeated over many tanks of fuel. About half of this increase was instant and then it crept up for a bit, presumably as the better detergents in the Shell fuel cleaned the muck out of the fuel injectors. That's a 5% increase in economy for a 1% increase in cost.

The car is a Passat diesel, and I know of other people who have found the same thing with other makes of diesel as well.

FWIW, I am unhappy if I don't average more than 50mpg per tank, sometimes this can be nearly 60mpg on a tank. The aircon seems to make 1 or 2 mpg difference. I find that being gentle on the accelerator is much more important when the engine is cold - having to stop at a few junctions early in the journey can make a surprising difference on the overall economy of a journey.

I registered with this Forum to find out if anyone had this experience and Bingo!
I do much the same - 70 M'way miles per day of economy driving. Current blend Tesco diesel goives me 48.5 mpg; Asda 49.1 mpg; this week Sainsbugs 49.6 mpg (and rising). I do wait for the 5p per litre reductions when my wife gets the vouchers so that justifies the use. I will try some Shell diesel soon and test out your theory. Thanks for posting.

hanoverbm

anewman wrote: »

A coating in the combustion chamber is likely to reduce efficency. The best thing that can be done to improve efficiency in the combustion chamber is to have the engine correctly tuned and the valve clearances adjusted correctly.

Also if it creates a coating in the combustion chamber, imagine what it's doing to your fuel pump, your fuel injection and the fuel lines between your tank and the engine. Housebuyer I would seriously urge you to reconsider your business idea and set about a viable alternative.

I have been using MPG CAPS for 7 months now and am very pleased with the results. I have experienced an increase of 23% in miles to petrol burned and in addition reduced the emissions by that amount. The manufacturers claim that the CAPS will also reduce all emmissions by 50 -75%, so we should all be using them. As to the comment that they may clog up the fuel pump and other system parts, they will not. The catalyst is only activated by the compression and spark within the combustion chamber, it burns hotter (by approx 100 degrees F.) - No it does not make your engine overheat, the thermostat takes care of that but the extra heat DOES burn off the carbon residue that coats spark plugs and valve seatings and abrades rings, so it also looks after your engine and gives more power. I have a number of friends who have tried this and, depending on the way they drive, they have all saved money. One managed to save 45% on his regular journeys. (think of that - a 50p liter) I calculated from Dept of Transport emissions tables that if all vehicles in the UK used these CAPS there could be a reduction of over 90 million tonnes of co2 per year. My CAPS also cost less that the money I save in fuel bills. As you may have noticed I am a convert and want to do my bit for the environment as well as my pocket. So carry on Mr.Housebuyer and spread the message you and I may save the world yet.
Imagine it, all you super salesmen, trying to sell a product that everyone needs, that cuts all emissions, AND saves all your customers money by reducing the amount of fuel used in everyday driving - it is so good that very few believe it and won't try it out. The previous reply is typical sceptisism - like a child saying "I dont like that" without even tasting his/her delicious cabbage or carrots.

dellboy102

I always accelerate slowly on my 1.4 nissan sunny, am I correct in saying that fast acceleration would actually be better for fuel consumption!?

colin79666

dellboy102 wrote: »

I always accelerate slowly on my 1.4 nissan sunny, am I correct in saying that fast acceleration would actually be better for fuel consumption!?

According to the RAC the most fuel efficient method is to accelerate quickly (without over revving) and get into the highest gear possible (for the speed) quickly.

dellboy102

oh right, so basically floor it, had no idea, all these years I thought me accelerating gradually was saving me money!, I'm sure I read that it was more fuel efficient as well, with conflicting information its hard who to believe, different websites say different things.