Tire Sizing Effect on Time

[Cyclone03]

I'm ready for my first 200TW tire. I'm also going to go from an 18" wheel to a 17". My brakes will clear a 17".

I'm not going to flare the front fenders to run the 275 tire I should be running so I'm limited to a 235 or maybe a 245 ,8.1" v. 8.9" IIRC.

Of course I want to run more tire on the rear,and I can.

Is there any kind of rule of thumb for tire size v. auto cross lap time?

I'm not looking at open track work ,just local auto cross , and Good Guys stuff with street driving to and from.

Car is a 68 Mustang, front end is all Open Tracker Racing , no bushings all roller, 1 1/8" sway bar, 620 springs -1.5 camber , 3.5 castor. Rear is 5 1/2 leaf -1" with a Fays2 Watts link Bilstien shocks all around. 54% of 3400 on the front. It also has an FE parked under the hood.

Sorry for the noobe question.

[Paul Huryk]

We suggest you run as large a front tire as possible on a front-heavy RWD car. So if you can fit a 245 or 255 up front, go for it!

As most braking and all of the turning is done by the front tires, a larger contact patch has it's advantages on the street and track. Having a staggered tire setup can give mechanical understeer that makes driving fast pretty difficult.

If possible (like on the 3rd gen f-bodies we manufacture wheels for), we suggest as close to a "square" tire setup as possible - same size on all corners. A car will be faster (and easier to control) with 265 tires on a 4 corners than a 245/285 setup - even though the total tire width is the same.