In this work an attempt is made to develop a decelerometer, an experimental facility or apparatus, for measuring the braking efficiency of vehicle. The first attempt is made to design, develop & analyse the mechanical decelerometer. The trials test results are taken on Maruti – 800 car and Maruti – Omni Van. From graphs, it is seen that the theoretical braking efficiency and the efficiency shown by Decelerometer are in good agreement. The percentage difference got is more than 14%. This shows instrument does not meets the exact requirement, armature reliability, parallax error in design of mechanical components like gear trains and scale and pointer arrangement. So to avoid all these error, the micro controller based decelerometer have been designed & developed, which was tested on Hero Honda CD 100 SS, FIAT Uno & TATA 1516 Bus. The results obtained are more encouraging than mechanical Decelerometer giving minimum error in the percentage braking efficiency, i.e. less than 3%.