Gas usage at the same RPM question...

[alank2]

Hi,

I was watching the instantaneous mpg gauge (the one that bops all around while you drive) and I was travelling on a slightly up and down hill road using cruise control. What I noticed surprised me is that even though the rpm stayed the same and I stayed in the same gear, same speed, the MPG gauge was going up and down. I guess I always thought that the engine would use the same amount of fuel at a specific RPM, but now I am thinking this is obviously a wrong assumption.

So, would it be true that an engine that is being pulled to a specific RPM by the transmission is using much less fuel than an engine that is pushing the transmission at the same RPM ?

Thanks,

Alan