Correcting Speedo for different rear-end ratio

[Jeff]

Someone put a different rear-end in my '87 GN, likely a 2.73 low 3.00 ratio, so now my speedometer is about 15 mph slow. This seems to be messing up my shift points, and TC lockup.
SO the question is where is the speedo gear located, and can I put the parts from the old tranny (which I think came from the same car as the diff) in to the new tranny ('85 BQ)?
Or am I way off?
Thanks,
Jeff

 

[MeanBuicks]

Use this page to help determine what speedo gears will fix your speedometer. That's the easy part.

Fixing the speedometer will also help with the lock-up apply timing as well, but your shifts points will remain screwy. If you've got a genuine TR transmission (BQ/BR), then I'd be hesitant to screw with the calibration. You're better off living with the bad shift points until you correct the rear. That way you won't mess up the transmission to make it work with a rear gear that doesn't need to be there.

 

[Jeff]

Thanks for the link!
Based on the notes on that page I think I could just remove the speedo gear from the old non-GN tranny and install it into the new BQ tranny. I'd keep the BQ gear in case I ever get the rear-end replaced with the correct gears.
Sound OK?
Jeff

 

[HOTTOGO]

Originally posted by Jeff
Someone put a different rear-end in my '87 GN, likely a 2.73 low 3.00 ratio, so now my speedometer is about 15 mph slow. This seems to be messing up my shift points, and TC lockup.
SO the question is where is the speedo gear located, and can I put the parts from the old tranny (which I think came from the same car as the diff) in to the new tranny ('85 BQ)?
Or am I way off?
Thanks,
Jeff

Are you sure that it has different ratio rear? Tire size (dia.) will effect speedo calibration as well. I would check both of them before making changes to verify what is the cause for sure.
To verify this (that your gears could've been changed), jack up the rear axle and support it with jack stands. With the transmission in neutral, rotate the rear driveshaft by hand. Count the number of driveshaft revolutions required for one revolution of the tire.
If the driveshaft turns approximately 3-1/2 times, you have a 3.54 ratio, etc. It's helpful to use a grease pencil or chalk to make a reference point on both the tire and driveshaft.

 

[Jeff]

granted that I haven't actually checked the ratio, but considering that it's a junkyard rear end (yellow paint marks typical of junkyard parts) and the tranny was a junkyard trans out of a station wagon or something weak I think it's fair to say that it's not original. The speedo seemed to be correct with the old non-turbo trans. I think I'll just swap the speedo gears.
Jeff