Zero emission road vehicles.

[Ray P]

Get a bike, you can then do more than the speed limit and not get touched for it!

[Ray P]

[shane]

if the revs are the same at 20mph in 3rd gear as the revs of 30mph in 4th gear isn't petrol consumption more or less the same?

No. In your example one would travel 10 miles more in an hour.
Hi t
MILES per gallon.

In order to propel you faster the engine has to work harder, so you would have your foot down more and use more fuel. If you’re doing a steady speed, fuel consumption will be more or less the same in any gear, within reason.

[Nick]

within reason

Drag increases with the square of speed, but I doubt that matters much between 20 and 30mph for most cars.

[pre65]

In order to propel you faster the engine has to work harder, so you would have your foot down more and use more fuel. If you’re doing a steady speed, fuel consumption will be more or less the same in any gear, within reason.

I don't agree.

At (for example) 1500 RPM in 2nd or 3rd gear the engine may use the same amount of fuel, but in 3rd gear you will go further so the miles per gallon is better. As said, wind resistance difference would be minimal at those speeds.

[Nick]

You are just asserting "the engine may use the same amount of fuel" you need to show that's true.

[pre65]

You are just asserting "the engine may use the same amount of fuel" you need to show that's true.

I did say "may", but there is no easy way prove that.

It may be different for diesel engines with high torque ?

[Nick]

but I have no means to prove that.

Then you have no reason to believe what you believe. If you look at a typical engine map



You can see for a given RPM, the VE (which is related directly to fuel economy) will vary with load. And for a given load, VE will also vary with RPM. That's why there is a gearbox.

I think the answer is "it depends" (both ways).

If going at 30mph used no more energy than going at 20mph, how would it be possible to restrict the speed to 20mph?

[shane]

at 1500 RPM in 2nd or 3rd gear the engine may use the same amount of fuel, but in 3rd gear you will go further so the miles per gallon is better. As said, wind resistance difference would be minimal at those speeds.

That’s not what I said, and at 1500RPM in 2nd or 3rd the engine will most definitely not use the same amount of fuel. I said that at a given speed you will use roughly the same amount of fuel regardless of which gear you are in. My car will run at 40mph quite happily in 3rd, 4th or 5th, and will use roughly the same amount of fuel in each gear. You seem to think that an engine uses the same amount of fuel at a given RPM in any gear, which is not the case. In your example, at 1500 RPM in 3rd you will indeed go further in a given time than at at 1500 RPM in second, but you will need to open the throttle further in 3rd in order to generate the power to go faster, and you will use more fuel.

At 1500RPM my diesel Civic is doing about 20mph in 2nd and about 60 in 6th. According to your theory, it should be using the same amount of fuel to cover three times the distance in the same time. Unlikely…

[IslandPink]

In the end it's all about mpg, from a climate point of view, and I intend to check that if I can find out how to manually re-set the trip MPG function on my dashboard, and find a quiet road somewhere locally.

[jack]

Not as simple as MPG (or KPL for us modern types). ICEs produce different levels of exhaust gasses depending on a whole bunch of things and it's these gasses we're concerned by. Running at a lower speed may possible change lots of things, maybe for the better, maybe not.

[Daniel Quinn]

It's not as simple as individuals agonising over thier choices .

The fact that prevailing ideology places emphasis on individual action ,suggests to me as a species we are doomed .

However d day is beyond the death of current decision makers and that's the problem.

[Nick]

Not as simple as MPG (or KPL for us modern types). ICEs produce different levels of exhaust gasses depending on a whole bunch of things and it's these gasses we're concerned by. Running at a lower speed may possible change lots of things, maybe for the better, maybe not.

And pollution from brakes and tyres.

[pre65]

If going at 30mph used no more energy than going at 20mph, how would it be possible to restrict the speed to 20mph?

A gearbox ?

Why then is the most economical speed (in terms of miles per gallon) more usual in top gear at main road cruising speed ?

[Nick]

A gearbox ?

Think, you have a engine that is running at a given fuel and air ratio producing a given car speed (say 20mph). If the car can run at 30mph with no additional change of inputs, why would it run at 20mph rather than 30mph?

Consider a ball sitting on a perfectly flat surface, with no friction between the ball and the surface. the merest gust of wind will move the ball, and once it starts moving why would it ever stop?

If you are bored, have a look at the complex options a third party ECU has to offer to try and get a steady idle. Once you add a load it all becomes a lot more stable.

Why then is the most economical speed (in terms of miles per gallon) more usual in top gear at main road cruising speed ?

Lets assume I don't know. Why would that matter? You are the one making a claim with no way of justifying that claim.

But I would guess that its because that's how its been designed. If you look at the ECU map surface I showed, it has a peak VE, I assume the gearbox and transmission is chosen so that peak coincides with the most common use condition. I would put my own money on the fact that a F1 engine is not at its most economical at 55mph, or 70mph, or anything like that.