I was wondering how wide I can go with my tires on my 06 XKR without the tire sticking out and looking stupid. I have 285's now and it looks good, but I think if it were wider it would look even better. What do y'all think?

 

If you have stock rims, I think the problem you'll run into is that they don't make wider tires that will fit the rims. I have 285's on the rear and 275's on the front, and that's about as wide as I felt I could go on my stock rims. I think it would look a little better with the tires spaced a little wider and I've thought about getting some wheel spacers. I just took my handy-dandy digital calipers out to the garage to measure the clearance between the tire and the inner edge of the wheel wells. On the rear I've got 22 millimeters of horizontal clearance. In other words, if the spring was compressed so that the tire was forced up into the wheel well, I'd have 22 millimeters of clearance between the sidewall of the tire and the inner edge of the fender. So spacers any wider than about 20mm would risk rubbing the edge of the wheel well on bumps. If you just want to put on wider tires with no spacers, then tires 40mm wider than the 285's would be the maximum limit (40mm would add 20mm to the inside and 20mm to the outside of the tread width). In other words, 315's would be pushing the absolute limits of what would fit in the wheel well. Even then I'm not sure if there's enough room on the inner edge of the wheel well.

On the front I have even less space. My 275's only have 18mm of clearance between the sidewall of the tire and the inner edge of the wheel well. Also, the front wheels can turn from side to side. If I was going to put spacers on the front I don't think I'd go any wider than 10mm, and 295's would be the widest tread that would fit.

My car is an XK8, not an R. I'm assuming the dimensions of the wheel well are the same.

 

The stock tire widths are pretty much the widest that you should put on the stock wheels. Even so, Jaguar usually puts a wider tire to wheel width ratio than other manufacturers do, like BMW. Wider tires may fit, but the sidewalls will bulge and your handling will suck. For example, the X308 XJRs stock tire size is 255/40/18, but that's a little too wide for an 8" wide wheel. I guess Jaguar does it for a better ride, compromising handling.

If you look on Tirerack under the specs for the tires, you'll see the recommended wheel widths for each tire size.

If you want wider tires, get wider wheels.