Have you calculated your total running cost per mile including loss of value, fuel, insurance, service/repair costs and fees? How large is the fuel cost compared to the rest?
I did that and came up with about 1/3:rd of the total costs assuming I will keep the car for 3 years total.
Loss of value: 1/3
Insurance, service and tax: 1/3
My conclusion is the main reason for saving fuel for me is actually environmental. I could do with an older cheaper car that has poorer fuel economy for the same mileage cost but what would be the point? Also it's the smallest car that I feel comfortable driving the freeway on a regular basis.