The stock 20" wheels run 255/35/30 and 285/30/20 tires. Has anyone tried bumping up a size larger on the sidewall size? Like 255/40/30 and 285/35/20s?

 

Size (width) of a tire depends on a width of a rim. I have 295/30/20 rears (on both Vulcan and Vortex rims).

You can easily try increasing side walls but bear in mind maths. 30 means 30% of width. So for your 285 (rear) it is 85,5mm vs mine (295) 88,5mm. 35 will make you close to 100mm.

The widest is XKRS GT with 305 tire. But I'm not sure if it is 305/30/20. If so - going to 285/35/20 would increase a size of a wheel by less than 1cm over XKRS GT.

 

The tread size from a 285/35 to a 285/40 would not change one lick. It'd still be 285 mm. The 40 IS a percentage, as you stated, but that is only for the sidewall height. The width of the tire is exactly the same, merely taller by 5% of the width. So in this case, the sidewall would increase FROM 100mm to 114mm. An increase of 14mm for the sidewall, times two. The overall height of the new tires would be 28mm more than the original, so one inch in overall diameter and exactly the same width. That'd be a 1/2-inch taller ride height, but fill in the wheel wells better, and actually make the speedometer closer to accurate.
Yes, you'd need another inch to use a 305 tire. I'd prefer 315/25. Doesn't matter the ratio, but I've got this all figured out if I can find someone to make them for less than $2000.

 

on my 2007,i had 245,40,19 and 275,45,19 on the back.The front will be good but the back is gone rub if you go anything past 40

 

Thanks. So with my 20", I'm better of sticking with the stock sizes, rather than take that chance.