Olson82Firebird

I recently put in a new speedometer cable in my 1982 trans am. I bought it without a working one. When I put the new one in, I noticed it almost always reads 10mph over the actual speed. I have 2 speedometer gauges and it did it to both of them. Can someone help me?

OrangeBird

iTrader: (1)

Your tires , speedometer gear in the transmission , or the gears in the rear end are the usual culprits here . Either your tires are too small (outer diameter) , someone swapped the transmission and didn't put the original speedometer drive gear from the old to the swapped in transmission , or someone swapped the rear end (or it's gears) from a lower to a higher ratio (2.73 to 3.45 for example)*

* don't quote me on exact gear ratio numbers , my numbers are an example only

Olson82Firebird

Thanks for replying
I know the out diameter of the tires is not too small, but the guy i bought it from put a different tranny or rear end (3:42 gears) in it. Is there a way to tell which one is causing the problem?

sofakingdom

iTrader: (1)

10 mph over WHAT "actual speed"?

Does it read 10 mph when it's really doing 0, 30 mph when it's doing 20, 90 mph when it's doing 80, etc.? If so, pull the pointer off of the speedo itself and put it back on where it points at 0 when the car is doing 0.

Needless to say, this probably isn't your problem, so the description you gave is WRONG.

If it has speedo gears in the transmission that are mismatched to the gear ratio in the rear end, it will read some PERCENTAGE off. Might be, let's say, 20%; like, it'll read 36 when it's really doing 30, 60 when it's doing 50, 72 when it's doing 60, etc. In that case you need to LOOK AT what rear end gears are in it, calculate the correct speedo gear ratio based on that and the ACTUAL tire radius (MEASURED distance from center of wheel to ground, NOT "calculated" from the "trade" tire size like 215/65-15 or whatever), purchase the closest actual gears you can find for your particular transmission (whatever it is) that have that ratio, and install.

Calculating speedo gears is quite easy. Use Sofakingdom's Magic Universal Constant aka SMUC, which is, 1000 revolutions of the cable per mile. ALL speedometers use this number. For example, you measure your tires while they are at normal operating pressure and temperature, NOT cold, and find that they are 13¼" from center of rear wheel to ground; and while changing the rear end grease as you will be doing this afternoon just like any other sensible person would be doing to a car they just bought, you observe that you have 3.42 gears (i.e. they're stamped 41:12 on the edge of the ring gear). Given those measurements and observations, your tires roll out 13.25 x 2 x pi inches per rotation, which is 83.25", or 6.938'; since there are 5280' in a mile, that means that your tires rotate 761.1 times per mile; multiply & divide that by the #s from the gears (multiply by 41, divide by 12) to find how many times the drive shaft turns per mile, which would be 2600.2; divide that by 1000 to find the speedo gear ratio that will give 1000 cable revolutions per mile, which would be 2.600. Then go find speedo gears that fit your transmission that come as close as possible to that ratio. In this example, if you have a transmission that has let's say 14, 15, 16, & 17 tooth drive gears and 37, 38, 39, 40, 41, & 42 tooth driven gears available, and the 39 tooth driven gear (15 * 2.600 = 39) is compatible with the 15-tooth drive gear, that combo would be perfect. You might not get that lucky, but you can usually get within a percent or 2, which is close enough that the difference as tires wear, or as the pressure changes, is greater than the residual error in the gears; meaning, you have just entered the "measure with micrometer, mark with chalk, cut with axe" zone, and any further improvement in accuracy is not noticeable or significant anyway. There IS such a thing as "close enough" even if not quite "perfect".