..a 1999 Wrangler SE, hard and soft top, 31" Dunlop Mud Rover's...he told me it got about 13 mpg, which I expected. I have been driving it all morning interstate/city, and getting about 9.5 mpg. My question is, do you think the mileage is off because of the enlarged tire size paired with the 4 cyl?
Welcome to the forum and Jeep ownership.
How are you determining gas mileage? GPS or odometer? If the original owner mounted 31" tires and didn't change the speedometer gear then the odometer will be off.
My 4 banger YJ got about 14 mpg, mixed driving with standard tires. (It was a 4 banger auto trans and couldn't get out of it's own way
How fast on the highway? MPG can drop horribly when speeds of 70 mph are reached - the Jeep is a lot of things, but "aerodynamic" isn't one of those things. Do a few runs at 55 mph, refuel, do the same runs at 70 mph and check the difference.
Also driving style is huge - my wife drives my Commander once in a while and she consistently gets less mpg than I do. The lady has a lead foot when taking off from lights
. The original driver might not have been as aggressive with the throttle as you are.
If mpg is important to you, consider putting in an inexpensive vacuum gauge - not difficult to do or expensive, the gauge will let you know when you're in the throttle too heavy for good mpg.
9.5 is really pretty low, even for a Jeep. You don't get much of an "up" because it's the four cylinder over a six cylinder - the 4 has to work harder to move the vehicle so the mpg isn't much (if any) better on the smaller engine.
Mileage aside, you're going to love
your new ride!
Have fun, be safe, don't wheel alone.