If you run 255/35R18's front and 285/30R18 rear your speedometer will be fine. I don't think you will notice a difference in ride either. This seems to be more dependent on the tire brand/model than whether they are 17" of 18".
BTW I have run the exact same wheels on my 1995 Supra.
Later, Steve
BTW I have run the exact same wheels on my 1995 Supra.
Later, Steve