I was reading that thread about the speed sensor location and it reminded me of a problem a friend is having. His car is a 95 Z28 M6 for reference. the problem is that his car reads 5 mph slow on the speedo but on his scanmaster it reads right on. He has 18" rear wheels which originally we assumed could have been the culprit but i know we got the car calibrated right in tunercat and the computer reads the speed correctly. so my question is... what sensor does the speedo read off of? or is there some gear involved?

 

I think it's just tunercat/lt1_edit that doesn't calibrate those values right. I've had several similar complaints about the speedo showing 5-10 quicker or slower.

 

My stock wheel/tire/rear 96 Formula with 2.73s showed 5 MPH slower above certain speeds.

At 72 actual, it shows 70.

But at 140 actual, it shows 135 on the dash...

And now when I'm running 58, it shows 70... But that's cause of the 3.42s that were just installed....

I'm getting programmed this weekend for the new gears...

 

did those guys have conflicting readings on a scanner like this case?

 

Actually, I'm not sure. I'm guessing they checked it with one of those radar road signs or against another car. Are you saying this car shows the correct speed on the scanner but not on the speedo?

 

Well when ur going that fast...wouldn't your tires expand slightly more than at lower speeds (taller tire in effect). That would be enough to have your speedo think you're going slower than u are.

Joe

 

You dont need no stinking speedo.

Brad, it could be a problem with the speedo cable or something in the dash. Logically if the VSS sends a transmission directly to the computer and when it is sent out each location reads something different then it would seem to me that the end link on the dash board is at fault. You might try your guage cluster in his car or

Maybe try to swap PCMs to see if Madwolf's theory is correct.

 

exactly. his speedo would read 65 when the scanmaster reads 70 and we even rode along side each other to make absolutely certain he was actually doing 70. he thinks it might have something to do with the speed sensor on the tranny putting out the incorrect frequency after he unhooked it to put a new motor in. it doesnt make much sense to me though b/c i think that sensor feeds the PCM.. not the speedo but maybe im wrong. he has since unplugged it to clean the contacts but nothing changed.

 

Also,
there's gotta be a reason why stock GM gear size isn't set to it's actual value in the programming. For example... 1997 Z28 M6 3.42 gears has 3.40 for the speedometer and 3.44 for the transmission in the programming.
You should try playing with these values a little to see how it will help. It will take time to test them but I'm sure you can get it to work right.

 

that is strange... i just checked it out in tunercat by putting in the stock tire size and gear for a 95 m6 but the new calibration wont quite equal the original. also, i didnt see a transmission input anywhere in tunercat.. must only be in LT1 edit i guess.

 

If the scanmaster is displaying correctly then it must be the speedo is off or miscalibrated. Speed sensor feeds the PCM, PCM feeds the speedo so it should all be coming off the same signal. My scanmaster and speedo read exactly the same (I was kind of surprized).

 

Yes, I also think there's something wrong with the speedo. I thought you meant that the speedo is generally off, not that it differs from the scanner's readings.
Before he swaps the speedo, you can send me his file if you want, and I'll set both speedo and tranny constants to the same number to see if this fixes it. I know Tunercat sets different numbers for these two...

 

this is my car that brad is talking about. The whole story is when my car was bone stock the speedo would read 71-72 and the scanmaster would read 70. After i got my 18 inch rims brad reprogrammed my pcm and my speedo would read 71-72 and the scanmaster would read 70. Then i did pulled my motor and tranny to do my rebuild. I had pcmforless make me a tune and then brad programmed it for my 18 inch rims again and now the car's speedo reads 75 when the scanmaster reads 70. I didn't know if a sensor got messed up, brad double checked the programming and it was for correct for my 18's i don't know what went wrong. any help.