My son has a 05 GT convertible with stock wheels and tires. The spec. sheet calls for 17x8J wheels and P235/55ZR17 tires. He would like to go to 20" bullitt wheels and corresponding tires. He would like the center cap to have Mustang, GT or Ford emblem on them. He wants to keep the current height as close as possible without any rubbing or interference. He would like them all the same size so they could be rotated.
Any and all remarks and suggestions will be appreciated.

Steeda UDP's, Steeda CAI, Magnaflow h-pipe with high flow cats, Hurst Competition short throw shifter and a custom SCT tune so far.

Thanks.
Falconsprint

My car shown in the link below.

http://i741.photobucket.com/albums/x...RightFront.jpg

 

Just get a lower profile tire. If the front is 235/55/17 now, find out what diameter the tires are and get the corresponding profile for a 20" wheel, it will probably be something like a 235/30/20.

Keep in mind though that 20" wheels will slow the car down measurably.

 

67mustang302
Thanks for the input.
You also said "Keep in mind though that 20" wheels will slow the car down measurably."
Why?
I assume because the 20" wheels would be heavier.
falconsprint

 

Rotating inertia. It requires power to overcome rotating inertia and accelerate a wheel, the faster you accelerate, the more power it takes to overcome the inertia...so also the more rotating inertia you have, the more power it takes to accelerate at the same rate(or faster, or even lsower).

Rotating inertia in a wheel is roughly defined by 2 primary factors....wheel mass, and how far from the center of rotation that mass resides, or radius. With the rotating inertia of a wheel, it increases directly proportional to mass, but increases exponentially relative to the radius. In other words, directly increasing mass directly increases rotational inertia.....but directly increasing radius increases rotating inertia as a square of the radius.

Since a wheel and tire package has most of the weight concentrated in the outer portion of the rim(the hoop) and in the tire....increasing wheel diameter moves the majority of the total mass out even farther away from the rotational center.....in other words it increases the total radius. While larger diameter wheels are usually a bit heavier, it has little effect on rotating inertia....it's the increase in radius from the center of rotation that has a much more profound impact. You could even go from a steel 15" wheel to an aluminum 20" wheel and REDUCE mass, but rotating inertia will increase due to the larger radius.

If you do the math, a 20" wheel increases rotational inertia over a 17" wheel by approximately 38% JUST from the change in radius....But when you factor in the mass of spokes and distance from rotational center, as well as the actual primary center of mass in the outer part of the tire not changing quite as much, you still end up with a 25-30% increase in inertia, perhaps more. That means whatever power the wheels already rob with 17's, they'll rob 25-30% more with 20's, not including the increased loss from extra mass.

That's why guys with 20's run slower under the same conditions and driving than they did with stock wheels.

 

That was an awesome and extremely detailed explanation how the change in the wheel diameter affects the performance. This is exactly what I needed to show my son. I already passed it on to him. I want to thank you very much for taking the time to reply.
falconsprint