Frank Grunthaler wrote a nice bit about this subject, I'll see if I can dig it out of my files.

alistair

On 2012-03-24, at 6:56 PM, Roland wrote:

> So, in driving along, with the brain wondering as usual.  How much more
> fuel is consumed in a lower gear vs higher gear at the same speed?
> 
> I ran a quick observation.  On level ground, at 45 mph, the RPMs in 4th
> gear were about 2400, and in 3rd gear it was about 3400.  (These are not
> absolute numbers since there are some mods on my van, and your experience
> may be different).
> 
> And being a fan of physics, and mechanics, and all things we'd like to
> understand better -- I was wondering, at this speed how much more fuel is
> consumed in 3rd gear vs 4th gear?  Now the immediate answer is none!  It
> takes the same energy to drive at 45 mph in 3rd or 4th.  The wind
> resistance is the same, the drive-line loss is the same, other friction
> like tires are the same.
> 
> It seems like the ECU is most likely properly calibrated to consider
> gearing (just through the senors).  In 3rd gear, it would inject less fuel
> per combustion, since there are more combustion events, and the required
> amount for each combustion event is less.  And in 4th gear, it injects more
> since there are fewer combustion events per time period, and so each
> combustion event is required to be stronger.
> 
> And of course the relationship is degenerative since at any speed the
> engine must have a minimum amount of fuel, so this question may not apply
> to really low speeds or at idle.
> 
> Again, the same energy is required to push the van at 45 mph in either 3rd
> or 4th.  I assume that 3rd gear will use more fuel, but why?
> 
> Thanks guys!
> 
> Roland