A vehicle gearbox serves for torque and speed conversion with help of rotating elements. Therefore the gearbox experiences periodic excitation forces with a fundamental frequency following the rotation frequency. These excitation forces give rise to corresponding periodic response signals, i.e. signals having content at the fundamental (rotational) frequency and its harmonics. Order analysis is an analysis technique which is used to extract these harmonic orders from the response signals. This article intends to use the order tracking analysis for gearbox fault diagnosis under variable speed conditions to compare between healthy and faulty cases by using order extraction. Finally, determine maximum Root Mean Square (RMS) as severity index. All the experimental data are acquired from vehicle gearbox which is tested on the laboratory stand equipped with three dynamometers; the input dynamometer serves as internal combustion engine, the output dynamometers introduce the load on the flanges of output joint shafts. The pitting defect is manufactured on one tooth of pinion gear on the output shaft. The results are obtained from practical experiments prove that order tracking analysis is a correct way for gear fault diagnosis under variable speed conditions. One can thus say that the order tracking analysis method has extracted narrow-band (order) information from the signal which is not revealed in the frequency spectrum.