I see. So those two sets of data actually could be not sync in the same way as sampled in time: double mPressure; // kPa (tire pressure) double mTireCarcassTemperature; // rough average of temperature samples from carcass (Kelvin) Could this result in 15% off when applying to the Isochoric process as Euskotracks found? Does the tire pressure output as spot value? (seems suggest as I read). that says its physics is OK, just data representation may lead a such "error".