Speedo gears

[Doug78]

I found the following question on NetVettes.com :

"My 81 has 50 series tires, and it seems that my speedo is off 10 mph.(Too fast) Does anyone know what gear I would need for the transmission, or how to calculate what I would need?
Steve "

I have a similiar problem in my vette, someone replaced the gear with the wrong one.

Corvette Central carries about 20-some different speedo drive gears how do you figure out which you need? Especially if you are running tires/gears other than stock.

 

[BLACKDOG]

speedo gears

Doug78 ~ know your problem as I am running 265/50's with the stock 3.08 gears and I'm indicating about 10 mph faster than actual speed, which can be overcome by running off the Tach, but it also registers more miles than are actually being put on the car.
I haven't had any luck in finding what gear to replace to correct the speedo, but may be going to a shop in a few weeks to have it looked at before this cross country trip on the 24th.
Look for a speedo shop and with the original tire height, the new tire height, and the rear end gear ratios, they should be able to dial you in...I hope
Good Luck.

 

[WALDO]

SPEEDO GEAR

I ALSO HAVE A 81 THAT'S ABOUT 10 MPH TOO FAST AT ABOUT
65 MPH. I HAVE 225/70/15 TIRES & AUTOMATIC TRANS &
AS FAR AS I KNOW NO GEAR CHANGES FROM STOCK. QUESTION
WHAT RPM SHOULD I BE AT 65 MPH & 75 MPH? ANY IDEAS HOW
TO CORRECT THIS PROBLEM? THANKS ANY HELP & IDEAS.

 

[yooperod]

I would think if you know how many teeth on the driven gear and how many mph the speedo is off . you can calculate the change needed. (And hopefully the driving gear won't have to be changed)
rod

 

[Doug78]

Here is the list of speedo gears for Corvette Central:

68-81 Speedometer Drive Gear Automatic 36 Tooth White 533620 8.95 Each
68-81 Speedometer Drive Gear Automatic 45 Tooth Light Blue 533621 8.95 Each
68-81 Speedometer Drive Gear Automatic 34 Tooth Light Green 533623 8.95 Each
68-81 Speedometer Drive Gear Automatic 37 Tooth Red 533625 8.40 Each
68-81 Speedometer Drive Gear Automatic 44 Tooth Gray 533626 8.40 Each
68-81 Speedometer Drive Gear Automatic 35 Tooth Pink 533627 8.40 Each
68-81 Speedometer Drive Gear Automatic 43 Tooth Purple 533628 8.95 Each
68-81 Speedometer Drive Gear Automatic 38 Tooth Blue 533629 8.95 Each

Big selection, but which one do I need? Hasn't anyone changed this gear while doing a resto? I like to do my own work so taking it to some shop doesn't sound good to me.

 

[yooperod]

you need to pop the gear out a get a tooth count( should be too bad of a job) This is To have a starting point. also the accurate amount of speedo error at a hwy speed.
Say It's reading 70mph when your doin 60. and you have 36tooth
gear for example.you need a 15% reduction.

60 mph
--------- = .857
70 mph

36 teeth
----------- = 42 teeth
.857

I'm sure some where there's a listing that goes by year, trans axle ratio, and tire size, But many things get changed over the years.
This worked for me when I changed to 3.55 gears.
rod