You forgot to take one thing into consideration. The designation 235 refers to the width of the tire. The profile (75, 70, 60, etc.) is the ratio of the height to width. That and wheel diameter is what dictates tire diameter. So, you can go from 235's to 285's if you go to a lower profile tire with out changing the speedometer significantly.

Most speedometers are not accurate anyway. The accuracy can also vary throughout the speed range.