mine gets about 22 highway 19-20 other...umm check ure tps throttle postion sensor on the right side of ure tb that might be ure problem. check ure linkage cable and all, a little grease on the cable woundnt hurt

let me know what u find dude

 

and ure AIS i forgot those models have adjustments

AIS is a adjustment idle speed control actuator that u can adjust with a simple wrench

 

Yeah, i have an 89' D100 v6, I feel your pain. But there is hope i haven't been able to find many parts, so i resorted to basic tuning. I advanced the timing cut and exhaust and made a homemade cold air intake. Be careful when you do the timing because if you advance it to much you will overheat. Also move the kick down lever back. This will raise your shift points and make the truck drive alot better. It's cheap you can pick up a few horsepower for free. Good Luck

 

I don't know much about engines, but my 1988 D-100 has a 3.9 that is very slow. I keep it floored while driving in town or on the highway. It takes about 2 minutes for it to go from 35 mph to 60 mph on the highway, keeping it floored the whole time. Also, does anybody know what my 3.9 would get for mpg's?

Sounds like your catalytic converter could be clogged thus blocking the exhaust system.