The Stovebolt Site Forums The Shop Area Driveline highway rear end gearing

84 Chevy 1/2T rear end in my 49 suburban. Don't know stock gearing. At 65 the 283 is turning up 3500. Would like to drop the RPM back to about 2500. What rear end ratio do I need? BTW I am running a TH 350 trans. Is there a chart somewhere I can refer to? Any help greatly appreciated.



Weeds

What is your tire diameter?

It's all about the math. I use THIS online RPM calculator.
You need to know your tire diameter, rear diff gear, transmission gear and speed.
In your case, the TH350 final gear is 1:1.
I'd guess your tire diameter is 27" (thereabouts).
Those numbers with a 4.10 rear diff would give you 3316 RPMs at 65 MPH.

Switching to a 3.08 rear diff will get your RPMs to 2491 at 65 MPH with a 27" diameter tire.
However, you will sacrifice some performance in all gears.

I helped a guy lower his RPMs once without spending any money. The switch on his tach was set for a 6 cylinder. He had a V8. When he set the switch to the proper setting, the RPMs went down. Always worth checking.

Lug, thanks for the reply, I will do the math using the chart you sent.
Thanks again.


Weeds

, The 84 rear end is probably a 3.73 ratio. That was common as they used an o-drive automatic. I think a 3.08 would be a little too high with a 283 engine, as the suburban is heavy. About 3.23 would be about right.