speedometer calibration

Marks89GT · May 8, 2011

I just picked up an 89 mustang gt. I was told that it is completely stock except that it has been painted and the exhaust has been replaced. I noticed it before i bought the car that the speedometer is a little off. Ill be doing 25-30 and it will read 40-45 mph. Ive searched the forum and havent really found anything. It should be stock so i should have stock wheels and a stock rear end and hopefully stock gauges. It is a 140 speedometer. what is going on here and does anyone know how to fix it? Thanks a lot guys.

No.11 · May 9, 2011

you should the gear ratio yourself to make sure. jack up the rear and put the car in neutral. then turn the wheel one full turn while taking note of how many times the driveshaft turns. that'll give you an idea of the ratio.

or perhaps someone replaced the speedo gear with one that doesnt match.

trbkrb · May 10, 2011

Sounds like it has gears, and does not have the correct speedometer gear. Do as No 11 said, 3-1/2 driveshaft turns would be 3.55 gears, 3-3/4 turns 3.73. Then you can pull the speedometer cable and see what gear it has, note the color and number of teeth.

Marks89GT · May 11, 2011

so i did what you guys said and when i turned the wheel. the driveshaft turned only 2 3/4 times??? and i pulled the speedo gear and it was red.

Marks89GT · May 11, 2011

sounds like i have 2.73 gears in there. which matches the sticker on my door. under AX it has an M.

No.11 · May 11, 2011

ok sounds like you have the red 21 tooth gear. what you need is the beige colored 19 tooth gear.

here is a chart showing what you need.
Mustang Speedometer Gear Application Guide - LRS

Marks89GT · May 11, 2011

i dont see which one it would be. I have an 89 with manual transmission with 2.73 gears. The beige 19 is for a 90-93 automatic.

Marks89GT · May 11, 2011

this looks what i need but i think its what i already have...

1983-95 Mustang 19 Tooth Speedometer Gear & Clip Kit, Manual Trans at LRS - Same Day Shipping!

trbkrb · May 11, 2011

Lot of info here, but if you do have 2.73 gears in a 89 manual car, stock it has a maroon 16 tooth gear on the cable. The drive gear inside the trans is a yellow 7 tooth gear. That is what my car had stock. All three have to match to get the correct reading. In order for the speedo to read fast, the rear gears would have to be numerically higher, turning the cable faster, or the speedo gears have to have fewer teeth, thus turning the cable faster. If you have the red gear, the cable would be turning slower, not faster. I would double check the speedo gear and count the teeth and if you can look inside the trans, check for the yellow gear. If everything is right, then I would suspect the speedometer is bad.

trbkrb · May 11, 2011

Another thing I just thought about, could they have replaced the gauge faces? The 140 speedo came out mid year in 89, maybe they put a 140 face on a 85 speedometer? That would make it read fast for sure.

Marks89GT · May 11, 2011

ok great thanks. ill look into that. and post it back up here.

speedometer calibration

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