Hey guys,

I have a customer who is having trouble with his speedo reading incorrectly (based on comparison with a GPS). I'm trying to figure out if his tires are non-standard, and want to know exactly what the TunerCat speedo-related constants mean. This is on a $EEB PCM if that matters.

In the first case, his car was moving faster (GPS, about 4 more at 60mph) than the speedo indicated. This was assumed to be happening because he switched from 235/70/15 tires to 235/75/15, and also was running a different rear gear ratio than the L99 PCM tune (he has an LT1 Roadmaster). The original PCM data was:

Scalar: 21
Scalar Fraction: 0.982
Scalar Scantool: 34.22
Time b/w Pulses: 43.61

So, I used the TC speedo tool to accommodate for the new gear and tire size, and it gave this data:

Scalar: 18
Scalar Fraction: 0.979
Scalar Scantool: 30.677
Time b/w Pulses: 48.889

However, that caused the car to run slower (GPS, about 4 less at 60mph) than the speedo indicated.

So, I thought my last effort would be to use the revs/mile constant in the TC speedo tool, rather than rated tire size. I had him measure actual tire diameter: 28.875". Converted to circumference (*pi) = 90.711. Inches in a mile (63360) / 90.711 = 698.483 revolutions per mile (am I correct so far?). So when I use that in the TC speedo tool, I get this data:

Scalar: 18
Scalar Fraction: 1.007
Scalar Scantool: 29.802
Time b/w Pulses: 50.332

You can see it is slightly different than the above data using the rated tire size of 235/75/15. Does anyone know exactly how these numbers go together to calibrate the speedo? Can you tell by looking at them whether the newest data would bring the speedo closer to reality (GPS)? It needs to read slower than it is currently (than the second set of data).

Thanks for your help!

 

You don't need to understand the constants to fix this problem. I do think that info is on the web, but offhand don't recall where.

Go to tirerack and look up the actual tire and get the manufacturer's revs/mi specification (a quick look at tirerack shows it should be in the 717-720 rev/mi range). Program the speedometer using that. Compare the speedo and the GPS, and if they don't match simply multiply the revs/mi number by the ratio of the two speeds to correct it.

First, though, make sure he's testing at more than one speed (like 40/60/80 mph) and seeing a constant ratio between the speedometer and the GPS, because otherwise the problem lies elsewhere.

 

I want to understand the constants so we don't have to waste more shipping costs with trial and error.

He's confirmed it happens between 60-80mph, 4mph less than the speedo shows (and 3mph less at 30mph on the speedo). As I mentioned, the revs/mile I got from my calculations was 698, indicating a smaller than advertised tire. Which would make the speedo read faster than true if it's expecting it to be bigger, correct?

So does it sound to you like using the revs/mile value will correct the problem?

Thanks for the reply, Steve!

 

Didn't realize this was a mail-order thing.

Your logic makes perfect sense. But keep in mind the result will be dependent on the accuracy of his diameter measurement. Even done carefully by rolling the car on a flat surface there's going to be a lot of error in that measurement.

Given that, let's see if we can't find how the parameters actually work and make some sense of the numbers you got out of TC. I'll see if I can dig anything up.

 

The one site I know has some info on these parameters is impalassforum.com and it's down (as usual). The B-body boards do have a lot of reports of tunercat speedo calculator not working right for those cars.

If it's off by 3 mph at 30 and 4 mph at 60 that's not a simple tire size problem and won't be corrected by changing the revs/mi.


Ignoring that for now, my tunercat calculates 235-75-15 to be 722 rev/mi, so using the 30 mph number, you would want to use (27/30)(722) or 650 rev/mi.

56@60 would indicate 673 rev/mi.

 

Thanks again.

Why do you say it's more than just a tire size problem because of the difference in errors?

Isn't that due to the speedo settings using various multipliers, so the lower mph wouldn't be affected as much as the higher (i.e., 30 x .95 = 28.5 and 60 * .95 = 57)?