Sularus,
I'll start by admitting I know nothing about American cars, but.....
Have you considered the lower power to weight ratio a smaller engine will give you?
As an example, I used to work at a place that had two delivery utes, same body, but one ran a 4.2 litre V8 and three speed auto, the other, a 3.3 litre Six cylinder with a five speed manual. On paper, the Six should have been better on fuel, but because it had to be pushed hard all the time it was consistently worse.
I'm not saying that's the case here, but it's better to think about factors like this first.
