Generally, expect 15-19 mpg combined (city and highway). However, you could see lower mileage in city driving or with a heavier load, and slightly higher mileage on the highway with gentler driving. The 4-cylinder engine will typically get better fuel economy than the 6-cylinder.
To get a more precise estimate, you should look up the EPA fuel economy estimates for the *specific* engine and transmission of the 2001 Jeep Wrangler you're interested in. You can often find this information on the EPA's website (fueleconomy.gov) or in the owner's manual.
