Best is to find a hill, that for the speed you want to travel:
- has an up gradient that matches the best BSFC of the engine and gearing; and
- has a down gradient that allows you to roll at the speed you want.
Go up the hill. Turn off engine. Roll down the other side at the same speed.
Acceleration = 0
(you may have a little trouble finding the hill, but it must be there somewhere)