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I have a 1987 Silverado 5.7liter TBI 2 wd w/700r4 trans ,tire size is 235/75/15 i believe it has 373 geads in rearend which is a 10 bolt i get around 18mpg on highway can i increase mpg by going to a lower gear ratio in the rearend and what can i expect ?I use this truck mostly on Highway and tow a 4x8 trailer 3-4 times a yearHas anyone done this ? Also my service engine light comes on after driving about 5-10 miles but resets when turned off any ideas? Thanks for any help that anyone can give me Duane
 

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I probably wouldnt go much lower than 3.40's, I dont think you'd gain much more mileage and it would be harder on the engine and trans. Have you tried synthetic lubes, open airfilter/airbox, good tune-up, higher tire pressures, and removing any excess equipment/weight?
18mpg on the highway is pretty good for that eng/trans combo, it was never known as an economy package.
 

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18 is prety good, I get abought 9mpg on my 89 tbi 350 3500. but i have no overdrive (used to be a light company utility truck).
i get the same on my 92, hoping to jump it up a bit with new intake and at least a catback exhaust

sythetics are great as far as lubrication goes but the problem with putting them in an older vehicle like that is if the seals werent done recently you might find a leak somewhere, they tend to leak alot faster then conventional oils. that being said i have a friend that has an 87 MR2 and runs a sythetic blend and he doesnt have any problems with leaks
 
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