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Rear gear ratios

mrayburnell5

mrayburnell5

1/2 ton status
Joined
Sep 14, 2008
Posts
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Location
hondo, tx
Hey: I got a 91 Blazer, and I was looking at the rpo codes, and if I'm right, this truck has a 3.08 ratio axle gears. I looked up an online gear ratio calculator and at 70 mph my engine is only turning 1585 rpm!? I have 33x12.5x15 and 4l60 auto , np 241. Does this sound right, I thought these trucks had 3.73 gears.
 
72 MPH with 3.08 according to the calculator.
3.73 puts you at 59 @ 1585 RPM

4L60 - 4th gear = 0.70:1
X-fer case 1:1
Gears 3.08:1
Tire = 33 inches
 
Well, whats 2 mph among friends. The point is, I could not believe what a dog this Blazer is, and now I guess I know.

I just looked, the 2 mph difference is because I put 32" for the tire.
 
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Probably doesn't have the towing package, eh? Z82 I think it is.

I looked at a '89 with a 350 a number of years ago that a guy on here was selling. Thing had 2.73 gears in it...wtf GM?
 
No kidding, the only reason I figured this out was I was looking at the edelbrock system that replaces the tbi,an you have to fill out this card to get your chip repro and they gave a bunch of rpo codes to look at.
From what I figured out from edelbrock is this:
NA5= FEDERAL
L05= 5.7 TBI V8
MD8= 4L60 AUTO
GU4= 3.08

I think I'm going to put 4.56 gears and 35" tires, It seems that would put the rpm where I'm used to seeing it, and give it some grunt.
 
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Don't bother regearing the stock 10 bolts though.

Also, research that Edelbrock MPI kit. I recall hearing a lot of people being less than impressed for their investment.
 
I hear you on the 10 bolts, I have a 14 bolt ff on the ground that needs to be on the Blazer. I'm thinking it has 3.73 gears, but not sure, it came out of a 90 to 95ish 3/4 ton.
 
I'm running 3.73 with 33's. That's the highest gear I would run with that tire. With your 4L60, you may want to think about 4:11 or lower. Regearing a 14bolt makes sense if it needs a rebuild anyways.

If you want to stay with your 1/2 ton - 6 lug set-up, check your local CL for a 10 or 12 bolt rear wit the gearing you want with a matching front. I went from 3.08's to 3.73 really cheap. The rear axle was a new rebuild I got cheap locally. I got the front a few weeks later. I helped pay for gas to get it down to me.

EDIT: The easiest method to get the gear ratio for the 14bff is to pull the cover and check the numbers on the ring gear.
 
mrayburnell5 said:
I think I'm going to put 4.56 gears and 35" tires, It seems that would put the rpm where I'm used to seeing it, and give it some grunt.
Click to expand...

I had a K5 with that combo :rolleyes: and I was very pleased with the performance, it was a "do it all" truck and the 4.56's and 35's were the perfect choice with a OD auto trans.
 
mrayburnell5 said:
I thought it would be too jt512. I can't belive they would put 3.08 with an over drive trany.
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All about LITTLE tires and gas mileage. 235/75R15's and 3.08 gears + OD trans = 350 loafing along at 55mph and up. They were desperately in search of gas mileage back then so they did some crazy stuff.

Not to mention you have to figure they never planned on anyone buying that particular truck towing much more than a fishing boat on a small trailer.
 
mrayburnell5 said:
I tried that AZ79K5Project, but I did'nt see any numbers that would indicate what ratio it has.
Click to expand...

The number of teeth for each gear should be stamped on the ring gear. A series of numbers followed by for example 43 14. Divide and you have your gearing: 43/14 = 3.07.

A 41 11 stamped is 41 divided by 11 so the ratio is 3.73

Another method is to turn the pinion. counting each revolution until one full of the axle shaft is achieved. Placing a piece of tape on the top of each will help keep things straight.

If you turn the pinion a little over 4 times to one axle turn - 4.11. Turn the pinion 4 1/2 turns to one axle rotation gives you roughly a 4.56 gear ratio. Hope that helps.
 
That does help, I'll look at thoses numbers again and see what I find.
I tried the turn the pinion trick, but the axle had been sitting on the ground and filled the drums with dirt.
 
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jt512 said:
I had a K5 with that combo :rolleyes: and I was very pleased with the performance, it was a "do it all" truck and the 4.56's and 35's were the perfect choice with a OD auto trans.
Click to expand...

If you never intend to tow anything this combo isn't bad. Whoever owned my truck prior to me had 4.56 installed on both front and rear axles.

I have since munched those gears. Busted three teeth off towing my boat. Switching to a 14 bolt SF rear axle and 5.13 gears for some extra towing power.
 
My '86 was that config. 305 with a Quadrajet, 700R4, and 3.08 gears. SLOOOOOOW.
 
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