What determines fuel consumption?

Just like them a little more "trim" than nature intended.

If you mean that this is the average for all cars, then this is a useless figure.

It'll depend on how you measure it, too. A car might manage 110 mpg at a constant 19 mph in third gear, but is this more efficient than 55 mpg at 38 mph... or 55 mpg at 39 mph?

Try fourth gear at 56?

And 50 would be better for the little dears' lungs? >;-)

You mean the 100 mph / 100 mpg advert?

It's possible that it could manage 110 mpg at 22 mph, or something.

But you could do 30 mph in top gear in the Maestro TD. Perhaps it was more economical in fourth or maybe third?

As is ours. Thanks to my OBD-II Scanner, I can see that on the flat, there's a small advantage to using fourth gear at 30 mph indicated. But as soon as you come to even the slighest of inclines, or you change speed, there's an advantage to using third.

On 11-Nov-04 08:56:10, Mark Hewitt said

Isn't the most important thing, how far down you have your foot depressing the accelerator?

For example if your foot was flat down in 5th and you were doing ninety, wouldn't you be better off (fuel wise) in 4th with your foot 3/4 of the way down, and running at signicantly higher revs?

Again, I'm only talking about fuel economy.

All the best, Angus Manwaring. (for e-mail remove ANTISPEM)

I need your memories for the Amiga Games Database: A collection of Amiga Game reviews by Amiga players

Yes, that's the simplistic view and for the most part, it's true, however... well life ins't that simple, yes?

The load on the donk. Load depends on the environment (temperature, pressure, humidity), the accelerator position, the quality of fuel, the viscosity of the oil, the speed, so on and so forth.

Taking Kermit as an example, because I'm bloody hundreds of hours of logged data with me nerding about fuel consumption.

Taking 30 mph indicated, there's a small - ~21.5% Throttle* in third, ~21% Throttle* in fourth - advantage to riding along in fourth rather than third. Calculated Load is pretty much about the same, ~44%.

*The OBD-II Scanner reports no throttle as being 16.9% and full throttle as being 94.5%.

However, as soon as you come to accelerate or you encounter a hill, in third gear the engine is much more responsive and you can accelerate at a given rate with less Throttle and significantly less Calculated Load. You'd expect this since at 30 indicated, the donk's churning over at 2,300 rpm rather than 1,850 rpm in fourth.

At a corrected 30 mph (disclaimer: when the ECU says the speed is 30 so the speedo is at 33), there's bugger all difference in it. ~22.2% Throttle for both. Go figure! Okay, you're pushing more revs in third relative to fourth so there will be more friction. But, Calculated Load figures are around 46% in fourth and 45% in third. So the engine is working harder relative to how hard it can work under those conditions...

What does the above mean? That for our particular car, at 27 and then 30 mph, there's very little in it in terms of efficiency. My consumption records appear to favour driving along at 30 indicated in third but it's not a statistically significant improvement in consumption. :)

Going back on topic proper, what stuffs fuel consumption is high load conditions. Labouring the engine (trying to go up that hill at 50 in top gear with 45% Throttle and 100% Calculated Load) or hooning up it in third (50 mph, 65% Throttle, 100% Calculated Load, accelerating well). Going up it in fourth at 50 mph 30% Throttle and 70% Calculated Load is better.

Accelerating also knackers consumption. Prolonged acceleration isn't good, neither is as-fast-as-you-can acceleration. Somewhere between the two is good. I can't be any more precise I'm afraid although my evidence acquired suggests that sticking to a Calculated Load figure of under 75% is better than adopting a free for all. Trying to stick to a lower Calculated Load results in you going nowhere... :)

I seriously doubt that. What kind or car do you drive?

Conor ( snipped-for-privacy@gmail.com) gurgled happily, sounding much like they were saying :

I'd be very surprised.

I came back from South Wales a while back in the XM (ton and a half of 2.0 turbo petrol, 150bhp, 4spd auto with torque conv lockup), at a nice steady

60, in convoy with a couple of 2cvs, one of 'em an older and slower one - which was flat at 60.

All the way from the Severn Bridge to Reading, I averaged just over 40mpg.

75mph? Nowhere *close*. Low 30s.

It's possible, but highly unlikely. If fifth is seriously tall such that the engine is labouring at 56 mph (like the Dodge Viper), fourth should be better. Or third.

It does seem a bit strange..

Angus Manwaring (angus@angusm_ANTISPEM_.demon.co.uk) gurgled happily, sounding much like they were saying :

Not *quite* as simple as that.

At 3/4 throttle, there may well be less fuel being injected on every revolution, but there's more revolutions.

I was down there today. Went to a small village just past Shrewsbury on the A488. Marc'll probably whinge now about unsuitable roads but seeing it was 20,000 litres of milk, I suspect some of it may end up in his cupboard soon.

I've found thats the case in my Rover 600, a 1.8VW Passat, a 2L Mondeo and a BMW 525

True, but because the gear ratio is different your engine needs to produce more torque to give the same torque at the wheels. The power should be about the same though (give or take a bit of friction). The efficiency of your engine at these two points (eg 5th gear 2000rpm and 3rd gear 4000rpm) will be different, and the 5th gear efficiency is not necessarily going to be better than the 3rd gear one, it depends on how much power you are asking for (ie how fast or how steep a hill etc).

As a general rule of thumb (from the engine efficiency diagrams I've seen), the most efficient point is doing the rpm that maximum torque occurs at (about 3500rpm?), and loading the engine so that about 75% of that torque is being used. If you are wanting to use more power than this, the most efficient point occurs at higher rpm, and if you are wanting to use less power, it occurs at lower rpm.

It depends what sort of driving you do mostly. If you drive on motorways mostly, the easiest way to save fuel is to drive at 60 instead of 80 or 90. If however you are doing stop/start driving, the best plan is to try not to brake. I don't mean going round corners at 80mph, I mean not going as fast when you *know* you're going to have to slow down.

Eg, if you know you want to go at 30 round a bend, don't keep going at 50 until you get to it and brake, back off earlier so that you reach 30 in time without braking at all. If you see traffic lights turn red, lift off

*immediately* to minimise the fuel used between that point and you stopping at the lights.

Your logic escapes me there. 110mpg at 19mph is going to be the most efficient, as you are doing 110 miles on a gallon of fuel.

It wasn't the turbo.

I think that data needs to be evaluated with some care. It appears to refer to average hourly traffic speeds rather than specific constant speeds. In other words at the low end of the speed range it will be a mixture of traffic jams when the speed is zero combined with sporadic movement. That's a lot of wasted fuel for very little forward motion.