new tires=bad gas mileage

Best gas mileage regardless of tire size comes from increasing tire pressure. Increases tire pressure will decrease tire drag and rolling resistance. Look up your tire inflation data for your car as compared with the max rating on the sidewall of the tire and go for a compromise somewhere between the 2. If the ride becomes too harsh, decrease pressure slightly until you find a nice comfort band with mileage you can live with.
I think someone had a pressure graph or formula posted to figure this stuff out pretty close. I run all my cars about 8# higher than auto mfg. reccommended pressure.