Just did a quick google search for your model and I'm assuming you have an auto. Most report 15 city, 19 hwy, 17 mixed. A heavier foot will reduce those numbers. Maybe you need to do a tune up?

'88 Raider 2.6L - SW Hubs, Weber 32/36, Off-road Pkg - Dead
'88 Monty 2.7L - 5" Lift, Weber 38, 33's, Dual Fans, Dual bouncy (more goodies to install) - Still Kicking.
'03 Monty Sport - XLS, BFG A/T's - Dead