[kameram]

I was wondering if running different sized tires on the fronts and backs would make that much of a difference. I have a 1994 gt and autocross it often. I'm looking at new wheels and tires and I was wondering if I could run a 255 on the front and a 275 or higher on the back. I've heard that this can induce understeer is this true?

[kechme]

I have a 1999 Cobra and run 275's on 17x9in Steeda Ultra Lites all around. A nice advantage to running the same size tire is that it's alot easier to rotate tires.

[GeneralGuy]

In all likelihood this will increase understeer, yes, since the rear will break away long after the front, so the car will plow. In a perfect world, all 4 tires break traction at the same time, allowing for a smooth and controlled 4 wheel "drift" once all useable traction is utilized.

Wikipedia actually has a nice page about this with some basic guidlines to what induces over/understeer here:

Car handling - Wikipedia, the free encyclopedia

Its a ways down, but its a good place to start looking into things. In a perfect world with a perfect setup, you'd probably want the same width tire at all 4 corners, plus it helps with maintenece as has been mentioned.

[limp1969]

On alot of aix cars they will run bigger tires in the back. I run 275 all around also. Seems that the higher hp cars will run bigger in the back. I am looking at a race car now that has won many races in scca and is logged. It is a 88 mustang 2800lbs 350 rwhp and has 335 in the rear and 315 in the front. This is not AIX legal. And 255 275 may not be in the class you are running.
Toyo r888s are a really good tire for what you are doing. I drove my car yesterday with 888s on it then swapped to street tires and brakes. My lord a good tire is by far the best mod you can do

[domindart]

So is it best to run all four same size on a car?

I have 245 in the rear, 225 in front now. As I recall, the 245 on all four just looked nasty, so I had them luckily swap the fronts out.

[domindart]

Right now Im thinking about running all four 225 size tires, the 245s rub a little bit on the exuast pipe, so I would rather just get a smaller tire than go and bend the hanger on the xuast pipe.

[limp1969]

you would need to have coil overs and scale the car and have a chassis shop set the tow and alignment to run bigger tires in the back. I think this is not as easy as it seems when you really get down to it

[kameram]

how far apart do the size differences have to be for that set-up to even make sense? I mean I would understand if the sizes of the front and rear tires were drastically different but for a small difference I don't think so...What do you think the largest size difference is before you have to go to that set-up is?

[limp1969]

imo any race car would need to be scaled before putting it on a track. And any change from there on out. In other words if I had a car that was scaled with 245 frt and 275 rears and changed those sizes to 275 all the way around the weight would change from front to back because of the angle change

And also i am no chassis shop but hang out at them alot. there is one next door to my shop and I am buddies with them.
Just put them on and try it are you road racing the car or is it a DD