Cluster show 15 km/h more than reality

[Skym]

The calibration will tell me if its a gts speedo?!

The calibration should be for a GTR speedo (printed on circuit board), but when speedo is checked on bench and is out, could be that it was swapped. Also I think they can test it with speedo cable to see if it's that. But there are different colour speedo faces, so it probably would be obvious if speedo part was swapped.

[Frankyy]

so you bought the car with the mines ECU and the speedo was off by 15-20km ever since you bought the car or only recently started happening?

if it's always happened, then it's might need a calibration, most clusters have a dial behind the cluster that can be adjusted manualy, the ECU is not stock and the speedo signal passes threw the ECU, if the ECU was stock the chances are your cluster would be acurate as well

If it's been off constantly and doesn't jump up and down like Skym said then I'd go with a quick calibration( crossing fingers that there is an adjustable POT behind your cluster like most clusters out there)

If it just recently started happening then disregard what I just said

[PL420]

I dont know if it might be related but when I installed my HKS turbo timer, the speedo wire was already splitted with a pink splinner with a wire (old apexi turbo timer maybe?). I splitted it again with the HKS splinner but after the splinnet and not between actual splinner and ECU. The tubo timer works fine but its doesnt read the kmh. I ALREADY had the decalibration of the cluster BEFORE installing it. So my point is maybe the old pink splitter is the problem ?!? Oh yeah, the ecu had been open too, 2 bolts been grinded off!

[Frankyy]

Old timer or someone had already tried fixing the speedo by taping onto the signal wire and grounding it out with a resistor in between, tried many different resistors to get the speedo correct and didn't have any success so just removed the resistor and gave up?

[h4X6]

kinda off topic but not...

what would be a good way to test for this other then driving beside a car that for sure has a correct speedo? anyway someone can do it by themselves? (perferably without a GPS unit as thats a bit hazardous to be driving at 120+ while staring at a GPS)

[PL420]

Ill check the calibration, the gears and hope to find the problem!