harris_i

I am about to make the transition from Michelin Cups to slicks for autosprints and time attack events, but confused about tyre sizes.

I have a 2004 GT3 with standard wheels, so the original tyre sizes are 235/40/18 front and 295/30/18 rear if I'm not mistaken.

Have been offered Yokohama A005 slicks with tyre sizes that are utterly alien to me. Basically, 18" tyres are being offered to me in the following sizes:

280/710R18
280/680R18
320/650R18
280/650R18
250/650R18
230/650R18

What the heck does all that mean?! Also, I live in a very hot climate, so it has been suggested I go for medium or hard tyres, rather than soft or supersoft - does that make sense? Whatever compound I end up with, they need to be a significant improvement over the Cups. What compound do you suggest?

Thanks for your help,

Harris

insite

width in mm / outside diameter in mm / inside diameter in inches

so a 280/710R18 is 11" wide, 28" tall and fits an 18" wheel. the 650mm height is roughly equivalent to stock (25.4") i believe.

harris_i

Thanks.

So are we saying that if my car is lower than stock (I have the Kussmaul settings), then 650 will be too big to fit in the wheel arches? On standard size tyres, the tyres on my car are quite close to the arches - I measured from the base of the wheel arch to the floor and it works out to around 635-640mm.

This would presumably mean I would have to roll the arches to accommodate a 650 which I am not prepared to do.

I guess I should look for a smaller size tyre.

insite

no, you want to keep the stock outside diameters (a.k.a. heights) unless for some engineering reason you've chosen to go with a different height. the stock heights for your tires are about 650mm. stay with this height, even if your car is lowered. to match your stock sizes in a slick tire, you need 235/650R18 up front and 295/650R18 in back.

if they don't have these sizes, opt for a slightly narrower tire. a tire that is 'stretched' on the rim will handle better. you could go with a 230/650 in front and a 280/650 in back.

Larry Herman

Pirelli P Zero makes the perfect size for your car, 245-645s for the front and 305-645s for the back. If you are going to autocross it, I would suggest running the D6 compound all around, as it is the softest. For track use, D5s on the front and D4s on the back work nicely.

harris_i

Thanks - this is very helpful. Perhaps I wasn't very careful in measuring the stock tyres. Looks like I will go with your second option, ie 230/650 and 280/650, as they don't have the former.

harris_i

Thanks for the info. The Pirellis are hard to get hold of out here, but it's interesting to note your comments re compound. Most of the usage will be on track in hot conditions, so a medium compound probably makes sense.