i have a 85 trans am and am going to be putting 4.10 or 4.11 gears in my car this year or over the winter, does anyone know of someone who makes gears to correct the speedo. Im putting in a chrysler 8 3/4 and my dad has a complete spare carrier, he has a 8 3/4 in his 77 ta and the carrier has 4.10's i wanna be sure that i can get gears to have the mph. I know their are ones for 3.73's thats guys and gals.

 

To correct the speedo, you will need to change the drive and driven gears in the Transmission. The following information is given with the assumption that you're using a 700R4 transmission... I don't know how much different it is for a different transmission. Depending what size tires you have, here are your two options. The gears colors change so the gear you get may not be the actual color listed but the tooth count will be the same.

Option 1 (Tire Diameter 29.0)

Drive Gear = (p/n 8642620) 15 tooth (gray gear)
Driven Gear = (p/n 9780470) 44 tooth (Dark Gray gear)

Option 2 (Tire Diameter 28.0)

Drive Gear = (p/n 8642620) 15 tooth (gray gear)
Driven Gear = (p/n 9775187) 45 tooth (Light Blue gear)

If you like numbers and formulas, here's the formula used to determine which gears you need.

#Drive Gear Teeth X Axle Ratio X (20168/Tire Diameter in inches) divided by 1001 = # Driven Gear Teeth

Example: 15 teeth drive gear, 3.73 axle ratio, 28" tire diameter.

15 x 3.73 x (20168/28) = 40300

40300/1001 = 40.26 or 41 teeth

 

It has been a few years since I went thru what you are doing, but back then GM did not make a speedo "gear set" for the 4.10 that came in my car. 3.73 was as far as they went. There was an after market gear out there but it was like $135.00 !!! for one little plastic gear. I changed to a 3.73 and was much happier with that ratio anyway. Just a note if you have to change the gear on the output shaft of your tranny, like I did, it was fairly hard to get the old gear off. Good luck!

 

if i use larger tires esptires of that size it totally gonna kill my low end, someone must know of 4.10 gears, or a box that mine goes into to get the right speedo.

 

OOPS! I wasn't looking at the year of your car. If you have a cable driven speedo, and I think you do, There should be reduction boxes out there that will work for you. I wish I could say exactly were....... but a web search for speedometers should get you in touch with who you need to talk to. Sorry for the bum info earlier, that was for electronic speedos.