Just a guess here - I'm no expert - but I didn't think that there is a
strict 1:1 relation between engine rpms and vehicle speed.  I'm just going
on intuition here, but it seems that if you're going up a steep hill, the
amount of torque needed to overcome the gravity of the hill is going to be
greater than if you are on a flat stretch of road.  The greater torque
will require greater engine rpms to achieve the same speed.  It would be
easy to try an empirical experiment: find a hill near your house and 
start to climb the hill with the car maintaining a constant rpm and see 
if the vehicle's speed slows down. 
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