i have a 95 z28 that has 20 inch rims. the tires i have on the front are 255/35/20 and on the rear 285/35/20. i want to get 17 inch rims. i am thinking of torque thrust m's. they are 17x9 in the front and 17x10.5 in the rear, and i will go with a 275/40/17 tire all around. i have heard that bigger rims really slow you down. will switching to 17s and that size tire make me faster?

 

They SHOULD made you "faster". By that I mean smaller rims usually weigh less, which takes power to move them, which would make you faster off the line, but not top end. If you get some good quality light weight rims with some good tires you will notice a slight difference.

 

The rims themselves do not make much difference, but the tire diameter definitely does. Smaller diameter = more torque at the road patch = better acceleration. Larger diameter does the opposite. Top speed depends on how much power is available, too large tire diameter will prevent the engine from reaching optimum RPM and will not develop peak power, and the car won't be able to reach top speed. Too small a diameter will require more RPM for the same speed, and the engine may rev past optimum RPM and the car won't be as fast as it could. For top speed you need to calculate tire diameter, final drive gearing, optimum engine RPM, air resistance, and rolling resistance. Then match everything up so that the engine is holding at the power peak when at WOT. There in not much weight difference between 17" & 20" rims, but the lighter rim will allow faster acceleration. For top speed, wheel weight is of little importance, except for road holding, where less unsprung weight the better.

 

Smaller wheel diameter = more tire, and larger wheel diameter = less tire. You have to take the whole wheel/tire package into consideration when figuring your spun weight, not just the weight of the wheels.

 

is it that much of a difference because im trying to figure out if i should spend around a thousand dollars for new rims and tires?

 

Not that you're going to notice. I wouldn't be so concerned with the weight of them as much as I would be with their looks. Get what floats your boat.
Now, if you were strictly racing the car and every tenth of a second made a difference, then you would be concerned with the weight.

 

alright thanks guys. do you think going from a 20 inch rim to a 17 is too big of a difference? and can you have 17 inch rims and still be lowered because my car is lowered however im not sure how much because the guy i bought the car from didnt tell me.