* Engine size: The 1999 Explorer XLT came with either a 4.0L V6 or a 5.0L V8. The V6 generally gets better gas mileage.
* Transmission: Automatic or manual transmission will also affect fuel economy.
* Driving conditions: City driving yields lower MPG than highway driving. Factors like traffic, speed, and terrain all play a role.
* Vehicle maintenance: Properly maintained vehicles tend to get better gas mileage.
Expect a range: You'll likely see anywhere from 14-17 mpg in city driving and 17-21 mpg on the highway for the 4.0L V6. The 5.0L V8 will be considerably lower, perhaps in the 12-15 mpg city and 16-19 mpg highway range. These are estimates, and your actual mileage may vary.
To find a more precise estimate, you could look up the EPA fuel economy estimates for the specific engine and transmission of your 1999 Ford Explorer XLT. You can usually find this information on sites like fueleconomy.gov (for US vehicles) or by searching online for the vehicle's specifications.
