Gas mileage on a '78?!!

[Quik 64 1/2]

Going to a taller tire or bigger rim (15") will change your gear ratio slightly making it get taller. For example: I just went from 275 by 60 on the rear to 245/60. Now instead of 60 mph @3000rpm, I am now at 55mph @3000rpm. You must also take into consideration that wider (not taller) tires create more friction with the road causing fewer miles per gallon. Just take all these factors into consideration before making your decision. There's not much you can do with 13" rims as far as tires so I would suggest going up to 15" if possible