If you're driving at 35 mph in 4th gear at 2500 rpm are you getting the same mileage as driving 45 mph in 5th gear at 2500 rpm?

 

Generally no. While your aerodynamic drag at 35mph in 4th is lower so is the amount of ground you cover per rpm at nearly the lowest rate that the engine can consume fuel. It's not much harder for the car to go 45mph in 5th gear, so in almost all instances you will be getting better mileage.



Comparing two speeds in the same gear can be more interesting, at 35mph in 5th I might get good mileage. My best mileage ever (42mpg over a tank) was around 45mph. It drops fairly linearly from there until I get 36mpg cruising at 75mph. It can drop to 32mpg cruising at 85mph though, all in 5th gear.