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