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**Accelo** · 02-24-2023, 08:12 PM

2.482 * 3.08= 7.64 final drive ratio.

The 4L80 has the same 1st gear ratio as the THM400.
As the GM transmissions evolved first gear got lower, over time.
Will this work? Sure it will. But don't expect tire burning acceleration.
This will be just like the factory gearing if the transmission was a THM400 & 3.08.
Since you would have to run a transmission controller of some type as all 4L80 require a computer, to run, you might consider a 6L80.
First gear in the 6L80 is a 4.03 ratio. That times the final drive (3.08) gives you 12.41 ratio.
Second gear ratio, on the 6L80 is 2.36. It's so close to first in the 4L80 you can consider them the same.

10 to 1 ratio is considered an optional acceleration ratio. I prefer closer to 8 or 9 to 1 ratio.

To recap;
The 4L80 1st gear is a little high and the 6L80 is a bit low when using the 3.08 gears.
The 6L80 also has over-driven 5th and 6th gears so it drops the RPM at speed a lot.
The 6L80 is like a THM400 or a 4L80 with a much lower 1st gear and two overdrive ratios.

Your choice, guess it would depend on if acceleration or mileage was was the most important criteria, and of course cost.
Cheers

02-24-2023, 08:12 PM	#5
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,360	Re: Trying to figure out if I will need to regear my rear end 2.482 * 3.08= 7.64 final drive ratio. The 4L80 has the same 1st gear ratio as the THM400. As the GM transmissions evolved first gear got lower, over time. Will this work? Sure it will. But don't expect tire burning acceleration. This will be just like the factory gearing if the transmission was a THM400 & 3.08. Since you would have to run a transmission controller of some type as all 4L80 require a computer, to run, you might consider a 6L80. First gear in the 6L80 is a 4.03 ratio. That times the final drive (3.08) gives you 12.41 ratio. Second gear ratio, on the 6L80 is 2.36. It's so close to first in the 4L80 you can consider them the same. 10 to 1 ratio is considered an optional acceleration ratio. I prefer closer to 8 or 9 to 1 ratio. To recap; The 4L80 1st gear is a little high and the 6L80 is a bit low when using the 3.08 gears. The 6L80 also has over-driven 5th and 6th gears so it drops the RPM at speed a lot. The 6L80 is like a THM400 or a 4L80 with a much lower 1st gear and two overdrive ratios. Your choice, guess it would depend on if acceleration or mileage was was the most important criteria, and of course cost. Cheers Last edited by Accelo; 02-24-2023 at 08:24 PM.