Hi, I have fitted Vredestein Extreme 235/55/17 on my T31 speedo is not effected it still reads high against my SatNav so is still legal. This is the largest size you can fit on this width of rim.
The Vredsteins are cracking tyres, pressures are unusual though Vredstein recommend 36 front/39 rear. Pressures work, little or no understeer.
Hope this is useful.
Regards: Kelba1946