Do stock rpm gauge only work with the motor that's supposed to be in the car? The reason I ask is, I was driving my little brother's car (SCHYBRID, and BTW, his car is still pullin' hard as *****, as he would say), so anyway, I was drving it and I was hitting the rev limiter while the factory gauge read about 7000 RPM. The VAFC though was reading 8400. Is the stock one reading it wrong or just too slow to keep up cause the RPM's go up really quick. Just wondering......