95 z28 speedometer calibration

[LSVolvo940]

95 z28 , t56 , 4.10 gears and I am using TunerproRT . I tried to change the drive shaft pulses per revolution it is currently set at 44.023 pulses per revolution. I have tried playing with different numbers but cant seem to get it to read a higher speed. Not sure what exactly to change but if I could alter the speed sensor signal I would verify and adjust with a GPS. It basically reads 35mph when I am going about 55mph. Any input might be helpful.

[steveo]

did you try the speedometer correction spreadsheet to get the correct numbers?

there's a copy included with the EEX xdf on my website i think

[LSVolvo940]

I had not gotten around to fooling with the tune on the car until last night. Somehow I managed to make the speedometer read to max immediately, however I did find the worksheet for speedo CAL attached to the EEX file. So goin through the math's and everything I recalibrated the ECM and now it has no speed signal. Before it shot straight to 150MPH and hit fuel cut so I turned fuel cut off and got the kids to daycare before work this morning. Recal and now no response, only jacked up the rear end to check. I still have not ruled out a loose connection but I will post the values as well.

[LSVolvo940]

Speedometer Scalar Base * 11 PPM/4000
Speedometer Scalar Divisor * 10.8126 divisor
Speedometer Scalar Scan Tool * 51.543 divisor
Stopped Viehicle VSS Pulse Delay * 1.01 msec
Speedometer Scalar Driveshaft PPR * 17.040 Pulses

[LeMarky Dissod]

Here are the new values I came up with, maybe try these:
Speedo Scalar Base: 15
Speedo Scalar Divisor: 1.026
Speedo Scalar ScanTool: 54.996
Stopped Vehicle VSS Pulse Delay: 59.4msec
Pulses per DriveShaft Revolution: 17

[LSVolvo940]

I see where i screwed some of the numbers up. This was based on 7 surface feet at the tire so my calculations may show little different reduction. Going to go try your numbers and see if that works.

[LSVolvo940]

Works like a champ now, my speedo reads 55 at 65 though. Checked with GPS .

[LeMarky Dissod]

95 z28 , t56 , 4.10 gears and I am using TunerproRT.

275/40ZR17 tires?
If not, what size tires?

[LSVolvo940]

No, the rear tires are 245/50/17 when I got the car it had 285/40/17. I took those off the back and put on 215/40/17 at that point I put the 245/50/17 on the back those tires were on the front and looked dumb having taller tires in the front. Gps today says its going 67 with the speedo at 55 ish.

[LeMarky Dissod]

These should be closer because they're based on 245/50R17 (the previous was based on 275/40R17):

Speedo Scalar Base: 14
Speedo Scalar Divisor: 1.001
Speedo Scalar ScanTool: 52.564
Stopped Vehicle VSS Pulse Delay: 63.643msec
Pulses per DriveShaft Revolution: 17

[LSVolvo940]

I can try those numbers but if the math is based on the surface tire feet the calculation should be correct. Going off of tire size alone is erroneous for many factors. I never took a whole lot of time to figure this out, the car is a piece of crap but this has been a great help to me so far.

[steveo]

you'll never get a perfect speedometer. too many variables throwing flexible rubber at something. just aim for 'close enough for a cop'. even factory calibrated speedos with brand new OEM tires can be off by quite a bit.

[LeMarky Dissod]

First you said it read 35MpH @ 55MpH actual.
Next, it read 55MpH @ 67MpH actual.
My calculation used the wrong tires.
If it reads close, but slightly faster than actual, then that'll be good enough to help you avoid speeding tickets.

[LSVolvo940]

you can make it read right, and yes it does matter if it does not read right. This helps me and actually prompted me to disable top speed cut to get the car down the road. Now its way closer , looking forward to doing more with these cars.