would 18s slow me down

[V6Bob]

"I'm gonna end all your guys silly arguments, and explain why with a physics explanation. The torque required to turn your wheel is equated by moment of inertia times angular acceleration (T=Ia). Moment of Inertia, I, is equal to mass of the wheel times the radius squared. Now, you're trying to get the wheel to move your car at the same speed, so you have to apply torque to the wheels for them to turn. Since you have the square the value of your radius in moment of inertia, the larger radius has a much bigger effect on the amount of torque required.

Going from a 16" wheel to an 18"inch wheel, lets figure out the ratios of the two.

8^2 = 64
9^2 = 81

So, 81/64 = 1.2656."

This analysis needs a little work. Since most people keep roughly the same total radius (wheel+tire) when they change, the difference is nowhere near this much. The moment of inertia of the wheel is larger, but the moment of inertia of the rubber is less. Since metal weighs more, you still lose a bit, but nothing like 26%.

18s will make you slower. Without careful, repeatable testing, no one can say how much.

[95DGcamaro]

Originally posted by AlienInv8r:
I'm gonna end all your guys silly arguments, and explain why with a physics explanation. The torque required to turn your wheel is equated by moment of inertia times angular acceleration (T=Ia). Moment of Inertia, I, is equal to mass of the wheel times the radius squared. Now, you're trying to get the wheel to move your car at the same speed, so you have to apply torque to the wheels for them to turn. Since you have the square the value of your radius in moment of inertia, the larger radius has a much bigger effect on the amount of torque required.

Technically, that phsyics is only workable for a POINT mass and not a disk. With a disk mass it is actually = 1/2MR^2, so the difference is even less. The radius is near the same to keep the speedometer accurate (hence going from 245/50/16 to 275/40/17). So the small difference of radius is not affecting the wheels as much as the mass most likely is. Along with the mass, it depends on WHERE the mass is located. A lot of mass near the center will require LESS torque than mass around the outer edge (more tire mass on the outer edge on a larger wheel.. 275/40). So the torque required is MORE with bigger tires/wheels even if the weight is the same. May not be an enormous amount, but often times noticable.

So YES, 90% of the time 18's WILL slow you down.

-Scott