Alright, I have 295/35ZR18s on the rear. I am having trouble tuning for them. At 60 miles an hour on my cars speedo, the laptop running datamaster says im doin 65. the GPS says im at like 60.3 or some crap. So, does the cluster get a reading from somewhere different than the laptop/computer?

Also, after driving for exactly 15 miles on the GPS, my cluster odometer only reads 14.8.

To me, the car needs to be increased just a bit to be nuts on with the GPS and odometer and spedo. But why the hell is the laptop telling me 5 miles an hour faster than the cars speedo and GPS???

Thanks
Cody
(95 LT1, ODBI)

 

I would bet your gauge is just off - it's getting its signal from the PCM, as is the datalogging software. GPS is probably more accurate... I bet your tire size or something like that is affecting the PCM's calculations and the gauge reading just happens to coincide with the GPS measurement.

 

hmm. Well, why would I want the laptop to be correct, but my dash wrong? I dont stare at the computer while driving. Besides, the GPS is extremely close to the dash, NOT the laptop. that is why I am wondering why they are so different...
anyone else?
Thanks
Cody

 

the speedo get the same value from the ECU like your laptop!
But your speedo has a mechanical part inside e.g. the stepper motor for the pointer!
I think there is an inaccuracy in the mechanical section

The odometer has a stepper motor too - an inaccuracy is possible too.....

........compared with the GPS

 

Well I didn't say you WANTED it to be off... obviously you want your gauges to read correctly.
Both the gauge and software get the same data from the PCM, which ASSUMES correlation between its programming and reality, ie your tire size and gear ratio. Could either of those be different than stock? Underinflated tires?
The GPS tracking assumes things too... like the speed of electromagnetic waves. I think it's safe to say that that's pretty constant. Sure it can be off by some number of feet, but over 15 miles that is negligible.

 

I have 295/35ZR18's. My tuner and I are trying to get the acurate reading for the speedo now, that is why I am asking where each device gets its info, and why it would be so off. I just did a road trip, and after about 60 miles on the GPS, my car lost one full mile. That is just unacceptable. That is why I am trying to figure this bad boy out!
~Cody