Speedometer is not reading correct speed.
Thread Starter
Burning Brakes
Joined: Sep 2001
Posts: 1,190
Likes: 192
From: East Coast
Speedometer is not reading correct speed.
Speedometer is not reading correct speed.
Just recently reinstalled rebuilt M20 3885010 transmission with 8 tooth drive gear and 20 tooth blue driven gear. Rear end is 3.36:1
At 2500 RPM speed is showing 44 MPH. Should be 60 MPH. Speedometer is original to the car. Never replaced or worked on.
Prior to transmission rebuild transmission car had 3851325 transmission with a ratio adapter on the speedo cable.
Would have expected the speedometer reading close to correct with all correct components.
Do I need to replace or get my speedometer recalibrated?
Just recently reinstalled rebuilt M20 3885010 transmission with 8 tooth drive gear and 20 tooth blue driven gear. Rear end is 3.36:1
At 2500 RPM speed is showing 44 MPH. Should be 60 MPH. Speedometer is original to the car. Never replaced or worked on.
Prior to transmission rebuild transmission car had 3851325 transmission with a ratio adapter on the speedo cable.
Would have expected the speedometer reading close to correct with all correct components.
Do I need to replace or get my speedometer recalibrated?
Le Mans Master
Joined: Nov 2015
Posts: 7,342
Likes: 2,548
From: Really Central IL Illinois
If it worked correctly before you changed the transmission, I would look at what was changed, not what was not changed.
There are two driver gears for the shaft in the transmission that drive the plastic driven gear as you appear to know. I don't think that is it, but you might double check.
The other thing you had to change was the speedometer cable and sheath to eliminate the adapter. Is it possible the cable is too short within the sheath and not engaging the driven gear fully or you don't have the cable seated in the speedometer head or bullet in the transmission?
Just ideas
There are two driver gears for the shaft in the transmission that drive the plastic driven gear as you appear to know. I don't think that is it, but you might double check.
The other thing you had to change was the speedometer cable and sheath to eliminate the adapter. Is it possible the cable is too short within the sheath and not engaging the driven gear fully or you don't have the cable seated in the speedometer head or bullet in the transmission?
Just ideas
Safety Car
Joined: Sep 2007
Posts: 4,443
Likes: 326
From: Indy Indiana
in general, your cable is spinning too slow at any given speed. you speed it up by lowering the tooth count on your driven gear.
They are cheap, get you a 17, 18 and a 19 and try them all. calibrate with your GPS speedometer app on your smart phone.
https://www.ebay.com/itm/MUNCIE-4-SP...0AAOxyQ45Q~QOQ
or try this calculator.
https://www.5speeds.com/calculators/SpeedoCalc.html
They are cheap, get you a 17, 18 and a 19 and try them all. calibrate with your GPS speedometer app on your smart phone.
https://www.ebay.com/itm/MUNCIE-4-SP...0AAOxyQ45Q~QOQ
or try this calculator.
https://www.5speeds.com/calculators/SpeedoCalc.html
Thread Starter
Burning Brakes
Joined: Sep 2001
Posts: 1,190
Likes: 192
From: East Coast
Today I've procured 17 & 18 tooth driven gears. Hopefully I will get them before the weekend and give them a try.
Race Director
Joined: Mar 2001
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From: Mustang OK
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Do not make a mountain of a molehill.
Simply replace the plastic 20teeth gear with a 18 or 19 teeth gear, as already mentioned. With the difference you have mentioned, from before and after changing the transmission, I'm thinking most like an 18teeth gear will be the choice.
They are just TOOOOOOOOOOOOOO EASY to change.
