It depends on the truck configuration.

What do you have 2WD or 4WD? Auto or Manual?

My '89 trooper with auto only gets about 17 if I tool around
like a baby. If I pull anything or run it very hard, you can
expect somewhere in the 13-14mpg.

The manual 1st gen Troopers can get upwards of 20-22mpg.

4WD auto Pickup probably not so good... as they weight
almost as much as a first gen Trooper. 2WD manual, I'd
expect mid to high 20's, if taken easy.

1995 Trooper LS auto 3.2 DOHC /w SOHC intake
1989 Trooper 2.6 auto
1989 I-Mark RS DOHC 1.6
1991 Stylus XS DOHC 1.8