Final drive ratio info
- Thread starter s trout
- Start date
The 3.21 is the planetary gearset from the 66-67 Toronado. It has an 8-bolt housing. I have a friend in the GMC Nor'easters who has one, and when I asked about the rumor that it will behave like a limited-slip diff, he said that he tested it and it will do so under low-torque conditions, thereby delivering improved traction. He noted that if you gun the engine, the effect is lost and it behaves like a regular open diff where only one wheel spins.
All the other final drive ratios use the same 10-bolt housing. There are two ways to tell whether you have the stock 3.07 gearing or something shorter (e.g. 3.42, 3.55, 3.70): Either open up the diff and count the teeth, or make note of the coach speed at different RPMs and use this calculator:
Transmission Ratio RPM Calculator
Use this calculator to see how different gear ratios will change RPM. Note: 1st Gear=2.48:1 2nd Gear=1.48:1 3rd Gear= 1:1 Stock Final Drive Ratio: 3.07:1 225/75R16 diameter is 29.3"
www.gmcmotorhome.org
One final possibility is that there was a set of replacement sprockets for the chain drive called "Power Drive" that would yield a 3.50 ratio using the stock 3.07 gears. It was originally sold by CasPro and later by Manny Trovao. While it does alter the ratio, it is not an ideal solution because it doesn't reduce stress on the transmission. Once other final drive ratios became available, Power Drive pretty much became obsolete.
See this for difference between the 3.21 and other 10 bolt differential cases.
Thanks for the great info, I am not very well versed on the GMC differentials.
I had looked at the power drive chain as I am going to get the trans rebuild by Manny and it is an $800 option. Does the power drive put a huge strain on the transmission? It is a lot cheaper than what applied gmc charges.
Anyone know a closer reliable shop around San Antonio, Tx for the trans/differential as the shipping is now getting quite high as with having to return a core also. Do these differentials have a corporate name to them and what replacement gears sets are available and from who?
Also, does any of the major differential manufactures make a limited slip for this. or is it just that one the applied sells?
I had looked at the power drive chain as I am going to get the trans rebuild by Manny and it is an $800 option. Does the power drive put a huge strain on the transmission? It is a lot cheaper than what applied gmc charges.
Anyone know a closer reliable shop around San Antonio, Tx for the trans/differential as the shipping is now getting quite high as with having to return a core also. Do these differentials have a corporate name to them and what replacement gears sets are available and from who?
Also, does any of the major differential manufactures make a limited slip for this. or is it just that one the applied sells?
There is a fellow over on Facebook who makes limited slip, but I can't remember his name. You can "try" a search on one or more of the many GMC groups there.
In the absence of a lockup clutch for the torque converter, we cannot use a "1" for the transmission ratio on Drive. I'm not sure what number to use, but it isn't a 1, and may lead to significant error in our calculations.
