I have a 69 Cutlass w/TH400 trans and 3.23 rear. I have P215 70R 14" tires. Previously it had a 38t gear which was off by 20mph at 60mph. I purchased the correct gear housing so I could put a 40t and up gear in. The CSM shows which gears to use but is based on old tire sizes. I did some tire research and took a guess with a 44t gear and I am about 5mph fast on the speedometer. I'm thinking I should go down a tooth or two. Can someone make a experienced suggestion. I happen to also have a 43t gear on hand. I would rather get it right than do it twice.
thanks,
Steve

 

 

Pull the speedo housing back out, and see what color gear is in there. Based on the speedo gear calculator on TCI tech page you need a 40 tooth driven with a 16 tooth drive. That should get you pretty close.

 

Here's an easy way to answer the question, without guessing, based on grade-school math.

And before you do this, use your phone GPS to determine the actual car speed versus speedo indication.

We'll say GPS showed your speedometer to be 5 mph fast at 60 mph. That would be 5 divided by 60 = 0.077 fast (7.7% fast)

Therefore your driven gear needs to turn 0.077 (7.7%) slower (it needs 7.7% more teeth).

You currently have a 44 tooth gear.

Increase 44 teeth by 7.7% (44 times one plus 0.077 = 44 times 1.077) = 47 teeth.

Whoops, we don't like that answer because the maximum number of teeth available is 45!

The problem comes about partly because the tires on your car are smaller than and have less load capacity than those the factory put on your car.

The solution will be to buy an adapter with ratio of about 0.923 to screw on to the transmission to slow the rotation. For example, look at https://speedometercablesusa.com/gear_box_adapters.html