[C2] Speedometer Calibration
Thread Starter
Instructor
Joined: Nov 2005
Posts: 101
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Speedometer Calibration
I have a 65 coupe with a 67 M20 4 speed transmission. The differential is stamped as a 3.36 and my tires are 27 inches in diameter. This car has always been running 15 mph slower than the actual speed. According to the on-line calculator, I required a blue driven gear. I replaced the gear but it had no effect on the speed reading. A visual count of driveshaft to wheel rotations indicated the rear may be a 3.08. I purchased the 18 tooth brown gear and installed it today. A test drive now indicates it is still 15 mph slower than my GPS speed. At 3,000 rpm, my GPS indicates 70 mph. This is telling me I have a 3.36 rear. The indication on the mechanical speedometer shows 55 mph.
I know this is a long winded lead up, but does this sound like a calibration with the speedometer itself? Can these be adjusted in some way and is this a common problem???? My changing of driven gears seems to have little positive results.
I know this is a long winded lead up, but does this sound like a calibration with the speedometer itself? Can these be adjusted in some way and is this a common problem???? My changing of driven gears seems to have little positive results.
Team Owner
Joined: Feb 2003
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From: Sitting in his Nowhere land Hanover Pa
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There are two diffreant driven gears in the trans that mesh with the plastic gear your replacing. If you jack your car up and count how many times your valve steam makes one turn in comparison to the drive shaft should give you a good starting point to the rear gear
Team Owner
Joined: May 2006
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From: CA
I believe the Speedo can be calibrated...who does that?
someone will chime in with a source...
Team Owner
Joined: Feb 2003
Posts: 54,104
Likes: 9,418
From: Sitting in his Nowhere land Hanover Pa
2024 C2 of the Year Finalist - Unmodified
2015 C2 of Year Finalist
Thread Starter
Instructor
Joined: Nov 2005
Posts: 101
Likes: 1
Re: Speedometer
My differential stamped code is: AM which means it is a 3.36. I treated it as such and installed the correct driven gear: blue. After my test drive, it continued to show a speed too slow which led me to think that perhaps the diff was changed by someone to a 3.08 at some point. Counting the wheel rotations to one axle rotation resulted in 3 complete turns plus about 1/8. So I thought maybe the 3.08 brown gear was the solution. The test did not reveal any correction. BTW, the cable is brand new too.
I guess I will have to remove the instrument panel and ship this speedometer out to a shop for overhaul and calibration. If this fails to correct the problem, I will simply live with it and simply mark my tachometer with small tick marks for speed reference.
Thanks for your reply guys; I appreciate your input and experience.
I guess I will have to remove the instrument panel and ship this speedometer out to a shop for overhaul and calibration. If this fails to correct the problem, I will simply live with it and simply mark my tachometer with small tick marks for speed reference.
Thanks for your reply guys; I appreciate your input and experience.
Safety Car
Joined: Feb 2006
Posts: 4,142
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That's a lot of work! You might be able to correct it with a ratio adapter.
http://www.bobsspeedometer.com/1/120/ratio_adapter.asp
http://www.stl-vettes.com/65vette/gearcalc.htm
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Speedo Gear in Trans
You should be able to easily change the Driven Gear, to correct your Speedo to the GPS Speed....These Driven Gear are inexpensive and should be easy enough to change to the corrected "tooth number".
Count the teeth from the original gear in the trans, and get the gear with required tooth count.....1 bolt and bullet comes out....
Pic shows a number of colored gears with diff tooth count...