http://www.edmunds.com/used/1991/toy...855/specs.html
The 1991 Toyota 4Runner has a 17.2 gallon fuel tank.
Assuming that you can completely fill the tank and also use the entire 17.2 gallons of fuel, how far you can go will depend on your fuel economy. Fuel economy will depend on how well maintained the vehicle is, and more importantly how/where you drive. Also, the fuel economy will depend if you have a manual or automatic transmission, the 4 or 6 cyl engine...
EPA fuel economy for the 1991 Toyota 4Runner:
http://fueleconomy.gov/feg/bymodel/1..._4Runner.shtml
I'll use 17 MPG combined, as that's about the average of the combined figures for the different available versions...
17 miles / gallon * 17.2 gallons = 292.4 miles