Speedo calibration
Thread Starter
Racer
Joined: May 2009
Posts: 340
Likes: 0
From: Santa Rosa Calif
Speedo calibration
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
Safety Car
Joined: Oct 2007
Posts: 3,729
Likes: 267
From: Austin Texas
Corvette of the Year Winner 2017
2016 C3 of Year Finalist
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.
therefore you need a speedo gear to match the 3.73 rear end.
they list the gears by color to match the various ratios.
Safety Car
Joined: Sep 2007
Posts: 4,458
Likes: 331
From: Indy Indiana
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
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
Team Owner
Joined: Sep 2006
Posts: 31,274
Likes: 4,366
From: Westminster Maryland
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
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
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
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.
If your speedo is off but not the odo, the unit needs to be repaired.
Instructor
Joined: Jan 2006
Posts: 228
Likes: 1
From: Warwick New York
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