Bumped my tire pressure up from 35 psi to 50 psi (max printed on sidewall is 55psi).

Milage went up from 12 MPG to 17MPG and that included a day of wheeling off road and crossing a 3,500 ft mountain pass.

Bit rougher but worth the gas savings.
I have to assume flat highway would be a bit better.

I'm at a 1/4 tank and 305 miles on the 18 gallons I put in before we left for the weekend.

Of course the last 1/4 tank in these XJ's only lasts about 20 miles! <img src="/forums/images/graemlins/lol.gif" alt="" />