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Mike S.
Mike S., ASE Certified Technician
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Experience:  ASE Certified with 25+ years experience.
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94 gmc: swap..my truck runs on 36 inch tires..im on the highway..rpm

Customer Question

My 94 gmc sierra has a 10 bolt rear end, and i recently bought a parts truck to swap out the diffs to a 14 bolt, which is a heavyer duty rear end, will the front diff be the same as what's on my truck now and bolt up the same way, and will the axles on the front fit the diff? Another question i have is , my truck runs on 36 inch tires and the gear ratio is 3:73. What's happening is that when i'm on the highway and shift to overdrive ,I lose so much RPM's that the overdrive barely pulls my truck, then kicks down to the next gear. Before I install my new diffs, im going to have them rebuilt, and wondering what size gears I should have my mechanic install?
Submitted: 2 years ago.
Category: GM
Expert:  Mike S. replied 2 years ago.

First of all what is the truck you have now, a K1500, K2500 or K3500? Chilton says the front diff on the K1500 and K2500 is a GM 8.5" 10 bolt. The front and rear ring and pinion ratios should be the same. The rear diff you say is a 10 bolt so it is most likely the 8.5" as well.

What is the parts truck diff's? I know you said 14 bolt, but are they 9.5", 10.5"?

Here is a chart,

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If right now with those 36" tires and at 60 mph you are reving at 2,050 rpm with the 3.73 gears perhaps switching to like a 4.56 set will raise the rpm to about 2,500 rpm.

If the 14 bolt is a 9.5" you can get a set of 4.56 gears here http://www.drivetrain.com/parts_catalog/ring_and_pinions/gm_9.5_ring_and_pinion_sets.html#9.5

for $252.

If its a 10.5" here are the 4.56 http://www.drivetrain.com/parts_catalog/ring_and_pinions/gm_10.5_ring_and_pinions_14_bolt.html

for $292.

Now you are going to have to put 4.56 on the front.

Expert:  Mike S. replied 2 years ago.

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