I searched for an answer, but couldn't find one. I'm thinking I need to adjust the Road Speed Constant in the ECM program to account for the difference in Tire Diamater between my donor car and the Fiero. Donor car had 225-60x16 tires ( 26.6" dia, 83.56 circumference). The Fiero has 215-60-15 tires ( 25.2" dia, 79.16 circumference) . Donor ECM had Road Speed Constant set at 24149.33 pulses / mi.

My thinking is this. Fiero tires are smaller, so there will be more rotations per mile, so to keep the output the same, I would need a road speed constant that is greater than that of the Donor.

83.56 / 79.16 = 1.0555 x 24149.33 = 25491.42. So instead of the RSC at 24k pulses per mile, it will be at 25.5K pulses.

Does this make sense? Is this the correct way to adjust this in programming?

As always THANKS!

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'87 GT in process, including GA / Seville brakes, Poly Suspension, '95 3800 Series 1 SC ( 225 hp ) 4t60e.

Another way that worked for me.
Using one of the tire and wheel programs that you find on the net. The ones that will show the changes in offset, revs per mile and so on. The ones that show the difference when you change tire size and aspect ratio(50,55,60,65,70). Or you can look up the specs for the tires you have. A little more precise.
Take the rotations/revs per mile and multiply it by the number of teeth on the speed sensor reluctor. That will give you your road constant. In my tune, I had to change the road constant due to tire size and difference in the tooth count. 29 instead of 30.

Yes, if your wheels have a smaller diameter then there will be more pulses per mile. So you're on the right track.

quote Originally posted by cmechmann: Another way that worked for me. Using one of the tire and wheel programs that you find on the net. The ones that will show the changes in offset, revs per mile and so on. The ones that show the difference when you change tire size and aspect ratio(50,55,60,65,70). Or you can look up the specs for the tires you have. A little more precise. Take the rotations/revs per mile and multiply it by the number of teeth on the speed sensor reluctor. That will give you your road constant. I.

Chris, I used your thoughts to "back into" the number of teeth on my speed sensor at 30 ( Road constant / Rotations per mile on Riv stock tire). If I follow your thoughts, I should multiply 30 times the revs per mile on my smaller tire, which will give me a smaller road speed constant, not larger as I had thought. Guess it would help to know how the ECM is using that road constant in it's calculations.....

No problem, though. Easiest method is to get it out and drive it, fix it, and drive it some more!! Doesn't take long to burn a new chip ( or I could hook up my Ostrich) Hope to be driving by this weekend.

quote Originally posted by darbysan: I searched for an answer, but couldn't find one. I'm thinking I need to adjust the Road Speed Constant in the ECM program to account for the difference in Tire Diamater between my donor car and the Fiero. Donor car had 225-60x16 tires ( 26.6" dia, 83.56 circumference). The Fiero has 215-60-15 tires ( 25.2" dia, 79.16 circumference) . Donor ECM had Road Speed Constant set at 24149.33 pulses / mi. My thinking is this. Fiero tires are smaller, so there will be more rotations per mile, so to keep the output the same, I would need a road speed constant that is greater than that of the Donor. 83.56 / 79.16 = 1.0555 x 24149.33 = 25491.42. So instead of the RSC at 24k pulses per mile, it will be at 25.5K pulses. Does this make sense? Is this the correct way to adjust this in programming? As always THANKS!

Take the number of tire revolutions per mile TIMES the number of teeth on your speed sensor's reluctor and this will EQUAL the number of pulses per mile that needs to be entered in the PCM tune for the road speed constants.

HOWEVER, when working with most removable chip GM PCMs, you need to keep in mind the speed pulses per mile constants only impact the speed shown on the scan tool. It does NOT seem to impact the 4k PPM output for the speedometer.

The IP Divisor constant is the only parameter I've found in the 94-95 3800 PCM that has any impact on the 4k PPM VSS output calibration from the PCM. And to make matters worse, this only allows for a coarse change, not a fine change; with only a limited number of possible settings...

IP Divisor Settings:
0 = Divide by 1 (for a 4000 ppm VSS)
32 = Divide by 9 (for a 36000 ppm VSS)
64 = Divide by 7 (for a 28000 ppm VSS)
96 = Divide by 11 (for a 44000 ppm VSS)
128 = Divide by 6 (for a 24000 ppm VSS)
160 = Divide by 10 (for a 40000 ppm VSS)
192 = Divide by 8 (for a 32000 ppm VSS)
256 = Divisor disabled, no output

If someone else knows of a way to get a fine adjustment for the VSS out of this PCM, I'm all ears. OBD2 PCMs are completely different (ie: you only need to plug in the correct PPM road speed constant to calibrate the speedo).

-ryan

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More is more. Less is not enough.

Custom GM OBD1 & OBD2 Tuning | Engine Conversions & more | www.gmtuners.com

Thanks Ryan. Guess I was just hoping for an easy solution. My IP divisor was 128, which now makes sense. Maybe I'll live with the 5% error for now...

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'87 GT in process, including GA / Seville brakes, Poly Suspension, '95 3800 Series 1 SC ( 225 hp ) 4t60e.

quote Originally posted by darbysan: If I follow your thoughts, I should multiply 30 times the revs per mile on my smaller tire, which will give me a smaller road speed constant, not larger as I had thought.

You'll actually have more revs per mile with a smaller diameter tire. Just FYI.

You can do the math on that, it's pretty straightforward. A mile is 5280 feet, or 63360 inches. Divide that by your overall wheel circumference, and you get wheel revolutions per mile. So for example, the wheels from the donor car would have about 758 revs per mile (63360" / 83.56" = 758.26), whereas your Fiero wheels would have about 800 revs per mile (63360" / 79.16" = 800.4).

quote Originally posted by Blacktree: You'll actually have more revs per mile with a smaller diameter tire. Just FYI. You can do the math on that, it's pretty straightforward. A mile is 5280 feet, or 63360 inches. Divide that by your overall wheel circumference, and you get wheel revolutions per mile. So for example, the wheels from the donor car would have about 758 revs per mile (63360" / 83.56" = 758.26), whereas your Fiero wheels would have about 800 revs per mile (63360" / 79.16" = 800.4).

My Bad! I had written the Revs/mile down next to the wrong tires sizes ( reversed them). Good now!

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'87 GT in process, including GA / Seville brakes, Poly Suspension, '95 3800 Series 1 SC ( 225 hp ) 4t60e.

quote Originally posted by darbysan: Thanks Ryan. Guess I was just hoping for an easy solution. My IP divisor was 128, which now makes sense. Maybe I'll live with the 5% error for now...

So far I have not found a way of correcting for a 5% speedo error using a 94-95 3800 PCM.