speedo gears

no scott, i just pulled the sleeve this morning and counted teeth, do you really think that adapter will be accurate?

 

There are several people I know who are using the adapters. It is a mechanical device that has gears inside that simply reduce the output. It will be as accurate as your calculations. GM did use them on cars with 3.90+ gears as the speedo gear selections were limited for high numberical ratios.
The most reduction this company sells is 26.7% off. This means you get 73.3% of what comes out of your trans now. According to the info you provided, my numbers come up to you needing a 50.95 tooth drive gear. So, 73.3% of that would be 37.35 or a 37 tooth gear to put you just reading a bit faster. A 38 tooth gear would have you reading too slow.
You would still need to buy the 37 gear, but not the drive gear or sleeve as it will fit in with what you already have in the trans. A local trans shop could even have that gear as it was used in TH200/350/400's and was a common gear for stock tires and gear ratios.
No speedo is 100% accurate as they are always a bit faster/slower due to gear combinations. Also, as tires wear, their diameter decrease. You just need to be as close as possible.

Scott

 

hey scott, would you mind showing how you did the math with the info i provided,it's not that i don't believe you, but how ya did it.thanks t.c.:Smarty:

 

You can set this up in a spreadsheet, but here goes:
A 28" tall tire has a circumference (Pi {3.141593} * diameter) of 87.964604"
A mile is 63360" (5,280 ft * 12")
divide 63360/87.964604, you get 720.2897202 rotations.
Multiply that by 3.73 and you get 2686.680656 transmission rotations
now multiply that by 19 and you get the number of rotations of the speedo drive gear teeth... 51046.93247
Now divide that by a 37 tooth gear and you get the number of turns of the driven gear, or 1379.646823
The number of turns you want is 1,000 (60 mph is 1 mile in 1 minute and is 1,000 on the dash speedo head). So, you are way over at 1379.646823 multiply that by the reducer output of 73.3% and you get 1011.695016 which is about as close as you will get.
WHEW!! :Smarty:
Now you know WHY we needed to pay more attention in the math classes we tried to sleep through. :Brow:
Hope this clear it up.

Scott