robs68

I have a 1968 with the original m20 4 speed. My speedometer is off. My tire size O.D. 26in. and my gear ratio is 3.73.Does anyone have the formula to get the correct speedometer gear. Thanks

69Vett

I think 26" is app. stock tire dia. (26.2 is stock I think.)
therefore you need a speedo gear to match the 3.73 rear end.
they list the gears by color to match the various ratios.

joewill

you will need to know your speedo drive gear number of teeth inside your transmission that is pressed onto your tailshaft. it is the gear that meshes with your speedo driven gear at the end of your speedo cable at the tranny.
then you can mathematically calculate it and base the calculation on 60 MPH equals 1000 rpm of the speedo cable.

another thing you can do is determine your speed via GPS and determine what percentage off your speedometer is and then call these guys to build you a add on box that will increase or decrease your cable speed the correct amount.

http://www.redrider.us/Speedometer_Gears.htm


or you can try several driven gears ( they are cheap ) until you get close to what your real speed is.

for example:

http://cgi.ebay.com/ebaymotors/MUNCI...Q5fAccessories

Alan 71

Hi Rob,
69V was very close ... I checked to make sure and found that the original Goodyears were about 26.9" in diameter.
I'm sorry but I don't know if or how that may affect the calculation for the proper transmission speedo gear for your new tires.
Regards,
Alan

Mike Ward

If you speedo and odo are both off this can be corrected by changing the tooth count of the driven gear. I think a change of 1 tooth = 6 mph but am not sure.

If your speedo is off but not the odo, the unit needs to be repaired.

7T1vette

Actually, it's more like 3-4 mph per tooth change. Adding teeth will result in a lower speed reading (from previous condition); reducing number of teeth will cause reading to increase.

captl27

here is a old post that might help you out

http://forums.corvetteforum.com/c3-t...r-replace.html

Mike Ward

Yup you're right, not 6 mph.