Author Topic: Need help choosing a new rear end ratio  (Read 117 times)

Offline Joe-NC

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Need help choosing a new rear end ratio
« on: November 30, 2019, 05:44:14 AM »
I have an 8.5, 10-bolt rear end. It currently has a 2.56 ratio with a chewed up ring gear. Which is great because I couldn't stand this dog much longer. I have a TH350C tranny with a mild shift kit and no stall in the converter. The engine is an old 305 which I will swap at some point. I'm running 30" tires now and will probably never run bigger than 32". I hardly ever tow anything. If I do it would be a lawnmower trailer. I use it for work about 4-8 times a month which is 20 miles away. Occasionally I get on the interstate where I would be running 70-80 mph. I will never be taking long trips or off-roading. I want to go from 0-60 as fast as possible and still be able to run 70 mph for 30 minutes without the engine screaming too bad. Fuel efficiency is not a concern at all.

I've looked at the rpm charts, but I don't have a tach so I'm not really sure what range my engine likes. But as I said it will be swapped at some point. Maybe a 350 or possibly an LS with a 700R4.

So what gear ratio would you recommend for my rear end rebuild?

Offline 75gmck25

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  • 1975 GMC K25 Camper Special, 350/TH350/NP203
Re: Need help choosing a new rear end ratio
« Reply #1 on: November 30, 2019, 06:06:21 AM »
My K25 has 4.10 rear gears, TH350, and 31" tires, so it runs about 2800 RPM at 60 MPH.  It puts you in a good power band, but sounds awfully "busy" in an old truck with almost no sound insulation.  However, this was how it was set up from the factory and it works well when the truck is loaded or towing.

I would think you may want to go with about 3.73 or 3.42 gears, which are common in squarebody trucks.  However, you have to decide how much RPM is too much.


Online ehjorten

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Re: Need help choosing a new rear end ratio
« Reply #2 on: December 02, 2019, 09:08:23 AM »
I wouldn't go any higher numerically than 3.73:1 for your case.  I have a '77 K20 with 4:10:1 gears, no overdrive and 31" tall tires.  I can get on the interstate and go 70 MPH, but she is working hard and I can push it to about 80 for a short stint, but 70 is pushing it for cruising for a length of time.  I wish it had lower gears sometimes because that high RPM running coupled with the noise is fatiguing on the driver.  At 70 MPH it is turning a little over 3,100 RPM.  At 80 she is turning about 3,600 RPM.

With the same 3.73:1 gears it would drop it about 350 RPMs to: 2,850 at 70 and 3,250 at 80.

With 2.56:1 gears I would be turning about under 2,000 at 70 MPH, so bigger than a 1,000 RPM drop!  Run your truck on the highway at 70 in 2nd gear and you will get the feel for what it is like!  With 30" tires, 2.56:1 gears you will be turning about almost 3,100 RPM at 70 MPH in 2nd gear.
« Last Edit: December 02, 2019, 09:10:04 AM by ehjorten »
1991 V3500 - Gen V TBI 454, 4L80E, NP205, 14 bolt FF, D60, 8" Lift on 35s
1977 K20 Silverado - 350, THM350, NP203, 14 bolt FF, D44, Stock Lift on 31s
1969 Chevelle Malibu Sport Coupe - EFI350, THM350
1968 Chevrolet Step-side Pickup - 300HP L6


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Re: Need help choosing a new rear end ratio
« Reply #3 on: December 02, 2019, 09:25:11 PM »
Disregard if you have found your answer.
With the case you have 3.08 ring gear is all that will fit. next step is change out the case and RG P.
85 C1500 GMC SIERRA-donor