Who was trying this setup on a track?
It works?

what tire? for R888 255/35 & 335/30 works. For R6 245/35 & 315/30 should work. R6's run wide.

For r888
335 is too wide for my 11j rims

255/35 315/30 r888 will effect an error?

the rear diameter needs to be 0.5-1.0 inches larger than the front diameter so that you don't mess up your AWD differential.

R888's come in 245/35? A combo that may be suitable for you is Nitto N1 245/40 & 305/35. Toyo is a Nitto company (or vise versa) and the N1 and R888 are the same compound but different pattern. N1's are a little cheaper.

255/35 18 = 25 (635mm)
315/30 18 = 25,5' (647mm)

it is just 0,5 inches

am I wrong ?

I live in Finland
no Nitto in my country =(