Whatever you save on gas mileage you spend on repairs.

For example - My last old vehicle (in 1995) was a 1978 F150 with a 302.

Not so great gas mileage but I could fix nearly anything on it myself, the parts were cheap, so I didn't spend much on repairing it over a decade or so since I had it. It rarely ever actually went to a shop.

When I bought my 2003 F150 the gas mileage was better - but few things could be done by me (meaning labor costs were expensive it had to go to a shop) and the parts were expensive.