i was told by the all mopar trans shop that did my a518 that very few autos were non lock up by 92. i got her rebuilt, rear sprag unit was blowed up... she's shiftin good now, slams hard into OD like an 18 year old girl. idk if they installed a shift kit too....

 

All first gens have non lock up torque converters the lock up torque converters didnt start until 94 when they changed the body style and put the p-pump on them!

 

I filled mine up before a 250 mile trip. The counter had like 280 sumthin and I topped it off just under 27 gal with the gauge at 1/4 tank. Which puts me at like 11 mpg... However on my trip back, I made it all the way home on 3/8 of a tank. First 100 the needle didn't even move


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My 92 auto extended cab 4X4 gets between 16 and 17 and its stock.
My 91 is a 5 speed reg cab 4X4 and mildly modded, but gets 18 to 21 depending on winter or summer fuel.

 

you should be averaging around 23 or so with no load just hummin down the road... even with 35's you will experience some lag off the line but at cruzing speed you'll be turning lower rpm's ..... should equate to milage gain's.... unless you are lugging engine...then lower is obvious......... when loaded i would see around 15 or 16....
think something is wrong with speedo/vs fuel used math.
need to go to highway speedo chech go 60 mph and check every mile for at least 10 miles see what your trip meter comes up with as far as milage.... if you went from 31's to 33's every 10 mile you should loose around 1.0 miles ... i went from 33's to 35's and lost 1 mile every 10 miles..... so when figuring fuel economy every 100 miles add 10... or like 356.4 miles add 35.64 miles...you get the point im sure...
if ya need any more help just shout out..