Speedometer, PCM and GPS all different

[BSonoma]

I didnt see anything similar in a search. I have 3.23 gears. when I bought the car the speedometer was about 10% off. I figured the car has different gears. Did some math and figured it has 3.42. It doesn't... I put the 3.23 gears back into PCM and its back to being about 10% off. last night I did a quick side by side by side comparison. I set the cruise at 70 per the speedometer. I plugged in the laptop to see that the PCM was reading for speed vs. the speedometer. For kicks and giggles I had the GPS running as well. What I ended up with has me even more perplexed. with speedo reading 70, the PCM was bouncing between 65 and 66 and the GPS was a steady 62. What the hell? anyone else ever run into this? any ideas on how to calibrate everything? I'm stumped.

[SSlowBoat]

I didnt see anything similar in a search. I have 3.23 gears. when I bought the car the speedometer was about 10% off. I figured the car has different gears. Did some math and figured it has 3.42. It doesn't... I put the 3.23 gears back into PCM and its back to being about 10% off. last night I did a quick side by side by side comparison. I set the cruise at 70 per the speedometer. I plugged in the laptop to see that the PCM was reading for speed vs. the speedometer. For kicks and giggles I had the GPS running as well. What I ended up with has me even more perplexed. with speedo reading 70, the PCM was bouncing between 65 and 66 and the GPS was a steady 62. What the hell? anyone else ever run into this? any ideas on how to calibrate everything? I'm stumped.
pending how good gps is, I wouldn't go by it. my work van has a factory gps that reads 3mph lower than speedo. I would go by pcm speed. the gauge clusters in these cars are notoriously inaccurate

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[shownomercy]

pending how good gps is, I wouldn't go by it. my work van has a factory gps that reads 3mph lower than speedo. I would go by pcm speed. the gauge clusters in these cars are notoriously inaccurate

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Yea, my speedo cluster will not agree with PCM or GPS more often than not. Old car problems.

[Injuneer]

The PCM drives the speedo, so any difference between the PCM reading and the speedo would likely be a problem with the speedo, converting the pulse count from the PCM to a physical movement of the needle.

Assuming the PCM value reads 65.5 MPH (average of values you observed) and the GPS values of 62 MPH is correct, the PCM error is 5.6% higher than actual. Having 3.42 gears in the rear, and 3.23 in the PCM would cause the PCM to read 5.9% high (not 10%). Pretty close.

What trans - auto or manual? If auto what led you to assume 3.42?

[Fastbird]

What size are the wheels and tires you have on the car? That would easily skew things. FWIW there's always going to be a difference between the PCM and Speedo, I don't think I've ever seen one perfectly pair up.

[Stormyweather]

I take the difference between the three and drive (set my cruise control to 2 above the speedo) cause the PCM is told by the sender in the trans and tire size is a factor.

It's not how fast you mow. It's how you mow fast.

[BSonoma]

***UPDATE***
I sent all of that info to Solomon, and he gave me a tune to correct it. Speedo, PCM and GPS all match. All shift issues are gone.

I had factory tire and wheels on when everything was askew. I didn't ask Solomon for the details of what he changed, I was just happy it was working. The difference was causing driveability issues at WOT. It would no shift 1-2, until I lifted off throttle, and shift flare from 2-3. All of that was gone with whatever Solomon did.