Speedometer cable turns per mile

[ceecpa]

Getting ready to send our Series V speedometer to Nisonger for custom calibration. Using the "Push Test" we measured 1375 turns per mile (TPM) on 165-75x13 tires and 3.89 differential. Lettering on the speedometer shows 1042 TPM. In use the speedo indicates 3.4 miles over a measured 2.4 mile route, and indicates 60 mph at what seems to be ~40 mph. We think we have consistent results because dividing the larger TPM, distance, and speed by the smaller yields errors ranging from ~30% to ~50%.

Any similar results?

Thanks

 

[Tom H]

How did your speedo come to be so far off? Did you change axle ratio? OD?

That's a big shift from 1042 to 1375, unless you changed to an OD tranny. An axle change from 4.21 to 3.89 only changes TPM by 8%.

And to confirm you do mean you did a push test to determine speedo cable TPM, not drive shaft TPM.

Tom

 

[ceecpa]

How did your speedo come to be so far off? Did you change axle ratio? OD?

That's a big shift from 1042 to 1375, unless you changed to an OD tranny. An axle change from 4.21 to 3.89 only changes TPM by 8%.

And to confirm you do mean you did a push test to determine speedo cable TPM, not drive shaft TPM.

Tom

I installed an OD transmission. The push test was only to determine speedo cable TPM.

When we got the car in 2003 the speedo didn't work because of transmission problems. I've since installed a SV engine (it had a SII) and an OD transmission, but I had no previous experience with how the 1042 speedo displayed speed or distance.

 

[Tom H]

I see.

So if a 1042 TPM speedo shows 3.4 miles over actual 2.4 , then it seems to me that your actual TPM is 3.4/2.4 X 1042 = 1476

The accuracy of the odometer TPM is fixed by gear ratios, so if your measured mile test was done well, this 1476 should be the correct TPM, Unless the speedo has been modified. According to Jeff Howarth an OD speedo should be 1536 TPM for a 4.21 axle, so that would be about 1420 for a 3.89, so maybe with larger tires your 1375 is about right.

Tom