[ATaylorRacing]

The old EPA sticker that came on your Jeep would have read 18 city/20 hiway with the four cylinder stick. The new revised sticker that would come on it if sold new today would be 16/19. Using this as a ruler shows that you are in the ball park. As for the tires. If your Jeep did come with the 265/75/15s then they would be 30.7 inches in diameter. The 31 in tires will give you a 1% slower speedometer reading and throw off your mpg figures by that same amount. Your 18-19 would really be 18.18-19.19 and your 20.5 would be a decent 20.7 mpg.

I am wondering if you actually have a wrong idea of what the stock size tire was. All of my sources for the 2000 model list the following sizes, diameter, and I have added speedometer/odometer error in case your vehicle should those tires compared to the 31s:
Jeep SE
225/60/16 26.6" 60=69.9 16.5% off
205/75/15 27.1" 60=66.6 14.3% off
225/75/15 28.3" 60=65.7 9.5% off
Sahara
225/70/15 27.4" 60=67.7 12.8% off
225/70/16 28.4" 60=66 9% off
30x9.5x15 30.0" 60=62 3.3% off
Sport
215/75/15 27.7" 60=67.2 12% off
225/75/15 28.3" 60=65.7 9.5% off
30x9.5x15 30.0" 60=62 3.3% off

Where did you get the265/75/15 tire size from. That is DARN huge for the four banger!