[70Coupe]

My car hasn't had a speedometer for a while, so I didn't need the help. But now I do.

I have a TH350 with 3.23 gears and 245/50R16 tires. The drive gear is pink and I can count 6 teeth (but since it's a worm gear, I assume the 7th tooth is there). I had a 19 tooth driven gear and was way off - 55mph on speedo took 1min15sec for 1 mile. 25% slow. I put in a 21 tooth gear and it got better - 1min10sec at 60 mph to go 1 mile - 15% slow.

Here's the question - Do I need to go to a 22 tooth driven gear (which I don't think is available) or change the drive gear to an 8 tooth?

All the calculators I've seen (TCI, etc) say I need fewer teeth but they make the speedo even more wrong.

Any help will be GREATLY appreciated.


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Bill Helton
70 Coupe, Daytona Yellow, 355ci, TH350,
Don't know the rest - just bought it.

[This message has been edited by 70Coupe (edited 02-13-2002).]

[70Coupe]

Sorry, my math was backwards. The calculators are correct.

[RickB1B]

You're probably right they don't make the small gear you're looking for. I had the same problem and it took me about a year to get it right.

The bottom line is when you have no more small gear options you need to switch the diving gear on the trans shaft. The question is how much, this is where it becomes trial and error but fortunatly it sounds like you're pretty close already.

I don't remenber from your post which you need bigger or smaller but do some geometry and you can figure it out. Then go to the Chevy dealer and ask of the appropriate gear with the number of teeth you need. Here's where you will run into trouble. Every year car had a different size gear and you will have to have a parts man who is willing to spend some time looking through the different years until you get the right gear with the right number of teeth.

I have a 67 with a TH 350 and my gear was listed as a gear for a 71 Camaro, in the 67 parts book it was not listed. Good luck it just takes time. BTW when you get close there is a scale on the back of the small gear package which will tell you which small gear you need. Based on my experience if you off the scale only a little bit (about 10%) you will need a one tooth change on the trans gear

[70Coupe]

Thanks for your reply. Actually, my math was so far off (backwards) that the gear I needed is a 18 tooth and my dealer had it in stock. So, for $7.95 I'm fixed. It's still a little off, but I can live with it.

By the way, if anyone needs a GOOD parts dept, Newton Chevrolet in Chattanooga is great. They have one guy that is very into older Chevys, and will take the time find parts.

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Bill Helton
70 Sport Coupe, VERY Yellow, 355ci, TH350,
All new suspension, 3.23 posi

[This message has been edited by 70Coupe (edited 02-14-2002).]

[FrankP]

I think I have seen 22 and 23 teeth gears
in the National Parts Depot catalog. I'd check for you but I don't have the catalog here at work.

Give them a call.
http://www.npdlink.com/