How do you know what speedometer cable transmission gear to use?

nitronut said

I put in a new speedometer cable in my 69 valiant today. 727 trans. (Third try, the last two times the speedometer needle jumped all over the place.) Today's installation was a success as the speedometer needle moves nice and smooth. My problem is, the speed reading is way off. Says I was doing 80 mph when I was traveling with cars I'm assuming were doing the 45mph limit. Went to a car show afterward and a couple folks said I need to change a gear where the speedometer cable connects to the transmission. Anyone know how to determine which is the right gear to correct this issue? Any info would be appreciated. Thanks...

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Hey Nitronut,

if the speedo cables you were using were new, you may not have had the gauge side inserted properly. The end of the cable is actually square and if it doesn't seat properly, it will cause the jumpy readings you were getting. When seated properly, you can feel the clip on the cable lock in place. (assuming this is clip style and not screw in type|).

Your other problem is probably caused by speedo gear in the transmission. Have you changed the rear end gear set or swapped out the rear end assembly? Is the transmission original to the car? You can determine the rear end ratio by checking for a ratio tag attached to the differential housing or remove the center section and check for ratio on ring gear. or, if an 8.25 remove the cover. This is most definitely the most accurate way to determine the ratio. Another less accurate way is to put the car on a lift , put the transmission in neutral. Put a mark on a tire and the drive shaft. turn the drive shaft and record how many revolutions the tire makes with each revolution of the drive shaft. If one turn of the driveshaft = 3.5 turns of the tire, your ratio is probably 3.55-1. Now, check here for the proper color speedo gear for your transmission.

Mopar Speedometer Gear Selection Guide - MyMopar.com

Also note, tire size is going to affect speedo readings as well. Hope this helps.


Cheers!!