You won't find a single definitive answer. However, you can expect somewhere in the range of 15-20 mpg on the highway and considerably less in city driving (perhaps 10-15 mpg or even lower). The 4-cylinder engine will generally get better fuel economy than the 6-cylinder.
To get a more precise estimate, you'd need to look up the EPA estimates for the *specific* 1992 Jeep Wrangler model (including engine and transmission) in a vehicle history database or owner's manual. Even then, actual MPG will vary based on real-world driving.
