I'm a relatively new owner of a '69 Hurst Olds and trying to sort a few items. Original size G60-15 tires and 3.42 rear, what TH400 speedometer drive gear and what adapter ratio would be factory issue?
Thanks.

 

Hey Briane our cars came with F-60-15's not G-60's maybe you miss type the size. good luck

 

Of course you're correct Guy--my screw up, thanks for the correction. So with the original size F60-15 tires plus a 3.42 rear, what speedo gear and adapter at the TH400 would one expect to have?

 

Is your speedometer not reading correctly? Assuming your tire diameter is 26.5" your drive gear should be 15 tooth and driven should be 39 tooth. Check your adapter at the transmission. It will have a range of driven gears stamped on it. If it matches your needs it's relatively easy to order new drive and driven gears. Just be aware that when/if you change out the gears you will dump a LOT of transmission fluid so have a big catch basin ready.
Olds transmissions have a slinger in front of the drive gear which is easy enough to wiggle off. But most companies send drive gears from Chevy which needs a skinny retainer clip. Everything will work fine. While you're doing this, change out the tailshaft seals and the driven gear adapter seal. Clean the transmission cable to adapter really well too so it goes on clean and no leaks.

 

Thanks for providing all the good advice Allan. I actually haven't registered and driven this car yet to know how far the speedometer may be off, if any. The prior owner thought the gears were steeper than 3.42 (easy enough to confirm, although haven't done so yet). So if I change them back to the original ratio I was wondering what TH400 driven gear and adapter I will be needing, Start looking for those two items now.

 

Easy enough to check the gears. Chances are the differential needs a fluid change. Take off the cover and look for the stamping on the ring gear. That will give you the ratio. This is the ring gear in my 72 Cutlass with 3.42. The numbers you want to see should be around the same as this for Olds. (12-41) Divide the 41 by 12 to get 3.41666 or 3.42. Sorry the picture is a little blurred, my fault with the camera back in 2012