Close, but not quite right on reading what the tire size means Chuck A.The speedometer is read off the front tire so the rear doesnt change it at all, just the engine rpm's per mph. On the tire size, the first number is the wideth is millimeters, second is the tire height is mm's, third is the wheel size in inches. The second number on the rear went lower which means you put a smaller diameter tire on, hence raising the rmp of the rear tire as the front stays the same. The wider the tire the lower the height is regardless if the wheel wideth stay the same. But as for the how far off is the speedometer, mine is 5 mph off at 55 acording to my GPS.
Second number is sidewall height expresed as the percentage of tread width.
Sidewall height of a 150/90-15 tire would be 150 mm X 90% = 135 mm
Move up one size to a 170/80-15 and the sidewall would be 170 mm X 80% = 136mm.
A 1 mm difference in sidewall height (2 mm difference in tire diameter) should be so small as to be impossible to tell a difference in RPM at a given speed.
The OEM front tire is 100/90-19. 100 mm X 90% = 90 mm sidewall height.
One size up is 110/90-19. 110 mm X 90% = 99 mm sidewall height.
The 9 mm increase in sidewall height is enough to notice a difference in speedo readings that most members report when changing up.