So, hypothetically, if new tires would increase your mpg by 2.5 (32 to 34.5), how many miles would you need to drive to make up the $800 the new tires cost?

You would gain an extra 35 miles per tank of gas (assume 14 gallon fillups at a cost of $2.50/gallon.) You would save $2.53 every 14 gallon fillup. So, you would have to drive 152,145 miles and burn 315 tanks of gas before you broke even.

Of course, you'd need to buy 2 more sets of tires to get to 150,000 miles, so, compounding the expense, make that 463,680 miles and 948 tanks of gas* just to break even on the first set of tires.

Personally, I feel mods are a bit foolish if the purpose is to save money. The gains are to be had in driving style.

*subject to fuzzy math rules

 

An update on the mpg search. I have since sold the 2.4 but had put a cobalt header and down pipe on it. At 65 mpg was 36.8. At 75 34.1.