Remember this also, low rpm doesn't always mean better fuel mileage, if that is what you are after. If the engine doesn't have the torque required at highway speed, you will be in the throttle a lot deeper then you think just to maintain speed. As you are calculating gears, figure in the sweet spot ( rpm ) the engine likes to run at. My 250 likes 2200 to 2600, it will cruise all day without any down shifting or much throttle.
