My 95 runs around 2500 at 70 with those sized tires. 75 is pushing it i thinking I'm turning almost 3000rpm Never have tried pushing to 80. I have 4.10s. But I do not use anywhere that much fuel in 25 miles. If I'm out on the highway I can get roughly 100 miles per quarter tank. I would look to make sure you don't have something else wrong causing the poor mileage.