[MrPogle]

It seems a given that max mpg occurs in the highest gear. I understand basic physics but I'm struggling to understand why this is. Can someone help, please?

Here is my rationale.

Cruising at (say) 60mph requires a certain amount of power. Gearing has no effect on power, so I need the same amount of power whatever gear I am in. In the old days, a caburettor was a relatively crude device and added fuel in response to airflow. It, therefore, made sense to keep the airflow as low as possible which meant low revs and so a high gear. With modern fuel injection the ECU is fully aware of gear, load, speed, throttle-opening etc so is not tied to air-flow like the carburettor. Surely it adds just enough fuel to generate the power required and no more. Therefore it should make no difference (within reason) which gear I use (e.g. whether I'm in 4th, 5th or 6th).

What am I missing here?

[gaza01]

engine speed...it takes more fuel for the car to rev faster

[djgandy]

It is kind of the same as why does a big engine use more power than a small engine. A lot of it comes down to internal friction.

It is a good question, and I've never found an answer I am happy with that ties in with the numbers you see on motor forums tbh. http://www.physicsforums.com/ is probably a good place to ask.

I do get your point though. For example at 60mph increasing speed to 66 mph in a higher gear may require RPM increase from 1500 to 1650 and that will take longer than raising from 4000 to 4400 in a lower gear. As you get there quicker you can take your foot off and use no fuel.

You will probably see though that at a given speed and a flat road the drop from 4400 -> 4000 is quicker than a drop from 1650 -> 1500 due to increased friction.

[doughboy]

+1 Lower RPM means lower frictional losses too and lower pumping losses which are a more significant proportion of all losses at light loads / lowish speed cruising.

At light loads, lower rpm will use less fuel as there are less compression strokes per minute.

OP you're right in that ECUs now can meter fuel very acurately, but they can't run it too lean or else it will over heat for example, so it has to maintain an optimum mixture ratio, if the engine is revving faster under light load you will use more fuel.

Although it is much less pronounced nowadays that it was with carb engines.

Forced induction messes around wth the theory though, as applying more throttle thus more boost at low revs allows much more fuel to be applied, giving more torque of course at lower rpms.

Then you will use more fuel at low revs than a lighter throttle at high revs.

[AlwynMike]

Originally Posted by doughboy

Forced induction messes around wth the theory though, as applying more throttle thus more boost at low revs allows much more fuel to be applied, giving more torque of course at lower rpms.

Then you will use more fuel at low revs than a lighter throttle at high revs.

Experience tells me that in a modern high geared diesel, it is just as economical (if not more economical) to run the car in fourth or fifth at 50/60mph, than to put it in top (sixth) gear

[MrPogle]

Thanks all for the info. It makes sense now.

[djgandy]

You may be interested in Brake Specific Fuel Consumption also.
http://en.wikipedia.org/wiki/Brake_s...el_consumption