hey jim ,i have some 19x8 wheels with +51 offset.
first off i have 235/35 19 tires on these wheels. is it possible to fit a 255/35YR19 tire on a 19x8 wheel?

also i have alexus is-f and thes wheels are as follows
19 x 8 in front / 19 x 9 in rear
Tire Size/Type 225/40YR19 front

 

I would stay with the 235/35R19 all the way around with those wheels. The 255/35R19 is going to be too wide for the wheel.
If I can help let me know.