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Speedometer not accurate...

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'66 Mustang (V6,Auto):  The speedometer is not accurate.  It hasn't been for a while.  My guess is that it's about 5-10 MPH below the true speed.  I'd like to fix it, but don't know how.  Please advise.  Thanks, Tom.

if you cant the tire size any at all will make your speedo incorrect. all you have to do is take it to a speedo chop and have the change the gears. it should be about 150 or more

make sure the gear in your transmission correspond to your tire and rearend gear ratio combination. a larger tire or a lower gear ratio can have huge effects of the accuracy of your speedometer.

1966 Mustang (Engine: 200 CID Straight-6, C-4 Automatic Transmission, Carburetor: Autolite 1100, Rear Axle: 2.83:1, Tires: P205/75R14, Style: Convertible, Color: Candyapple Red).
Where do I locate and how do I remove the driven gear?  I looked, but can't find it.  I saw online that the 66 Mustang original tire size is 185/75-14.  Is this correct? If so, I don't know if the speedometer was previously calibrated for that size or the current size (205/75/14). Thanks.


The driven gear is at the end of your speedometer cable. What you need to do first is see if the odometer is accurate. If your odometer is accurate, it's most likely the speedometer arm getting hung up.  Most states have a reasonably accurate milage marker along side major roads. Find a long stretch of road with markers, get up to 60 mph by your speedometer,  check the time elapsed between one mile markers with the second hand of your watch and the distance travelled on your odometer (remember the position of the tenths). At 60 mph, you should be doing one mile a minute and the odometer should read one mile.


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