Stock tires on a 1996 Jeep Cherokee Classic 4x4 are 225/75/15's I believe. I recently lifted it 3" and put 265/75/15's on it and thought I got really bad gas mileage but then realized that the bigger tire rotates less per mile which may only seem like I'm getting worse gas mileage because of the tire circumference rather than the engine having to work harder to turn them. My question is how do I calculate which gear I need to put in my transmission so that I can get actual MPH's and mileage?

Search this forum for "speedo gear". Look at the thread from Eseven. Find the link you need.

The way I do it is take the current size of your tire and divide it by the stock sice. So if you start with a 30" tire and upgrade to a 33" tire it is 10% larger. if your speedo says your going 75 mph then your really going 82.5 mph.