My question is that if i keep the same Rim's size but just shorter the side wall of the tires , how will it affect the speedometer ? Am i going faster than what i see on the speedometer or the otherway around

thanx

 

"Shorter" sidwalls will decrease the overall diameter of the tire. (Taller would do the opposite.) A smaller diameter of tire would spin more times over a given distance of road surface. Therefore, at any indication of the speedometer, your car would actually be travelling SLOWER than before with bigger diameter tires. Putting it in another way, your speedo would read faster.

 

you would be going slower than what the speedometer says. But, this would have a similar performance impact as shorter gears.