yeah 8 seems pretty low unless your hubs are locked or something like that.. i was averaging about 12-15 with mine.. sitting on 33x12.5.15 tires to boot. make sure you dont have anything dragging, for example if someone did a rear brake adjustment. theres a chance its set to tight. also blazer calipers on the front are notorious for not lasting as long as they should. if you have the Auto Hubs on the front axle. that could be a problem area also.. or if its switched over to a manual lock out.. make sure there set to free otherwise your pushing the axle/gears etc.. reseting code might take care of it also!!! also might want to make sure everything is greased... and that all fluids are in good shape. like diffs, and transfer case, tranny , engine oil etc.. if i remember correctly you said it had a crate motor, this might play a effect also.. if it has more ponies then stock theres a chance you might be stuck with a 8 mpg truck also..

hope this helps

Ck1500

 

well swapped out the heatercore along with the knock sensor, pretty sure it is just a stock crate motor and no extra HP going to reset the code and see if it helps. Also I have the 700R with overdrive but still seems to average 8 mpg on the thruway.