Gas mileage on a '78?!!
 

I have a '78 mustang II with a four cylinder, 3 speed automatic. the tire size is 155/80 R 13. my current gas mileage is about 10 miles per gallon. will bigger tires make my mileage better? Are these tires too small for this car? These are the same size tires as they were when I bought the car. Will a 215/70 R 14 or 185/ 75 R 14 be better or worse?

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

sounds like there is something wrong with your car.how does it run at idle aand while cruising,does it have any power?

... last thing i'd look for , would be a tire size improvement here... check the oil & see if it doesnt come up 95 octane....*shudders*