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**jocko** · 03-25-2014, 11:11 PM

what Tbone said - speedo being incorrect won't cause tach to be incorrect - it reads directly from the engine based on firing pulse.

If stock tire size and rear gear is the same, then the TH350 you swapped in would be from a vehicle that did not have a 3.08 rear originally. In other words, the speedo gear on the output shaft of your TH350 is incorrect for your setup. You can live with it (I had the SAME scenario in my 71) or you can have a trans shop change out the trans tailshaft speedo gear in order to correct the situation. If you have the old trans - IT will have the correct gear on the tailshaft, grab it! You can do this yourself also. Not exactly a fun job and you need to have some auto trans experience. Personally, I'd live with it til it's time for a rebuild. Just knowing what the issue is kinda makes it easier to live with...

EDIT: Also, there are speedo/trans mis-match compensators out there - old Stewart Warner units. I had one, tried it on my 71, still wasn't the correct ratio, so just left it off.
I was also going to post what BADAZ posted - the 2nd link is particularly useful.

03-25-2014, 11:11 PM	#11
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,995	Re: Speedo question HELP what Tbone said - speedo being incorrect won't cause tach to be incorrect - it reads directly from the engine based on firing pulse. If stock tire size and rear gear is the same, then the TH350 you swapped in would be from a vehicle that did not have a 3.08 rear originally. In other words, the speedo gear on the output shaft of your TH350 is incorrect for your setup. You can live with it (I had the SAME scenario in my 71) or you can have a trans shop change out the trans tailshaft speedo gear in order to correct the situation. If you have the old trans - IT will have the correct gear on the tailshaft, grab it! You can do this yourself also. Not exactly a fun job and you need to have some auto trans experience. Personally, I'd live with it til it's time for a rebuild. Just knowing what the issue is kinda makes it easier to live with... EDIT: Also, there are speedo/trans mis-match compensators out there - old Stewart Warner units. I had one, tried it on my 71, still wasn't the correct ratio, so just left it off. I was also going to post what BADAZ posted - the 2nd link is particularly useful.