maybe stuffed up? Should Ive said -4 inches / -10cm
17cm wide wheel,
place flat steel rod on back edge of wheel outer,
placed ruler inside supported on mounting face of
centre from the back 10cm deep to where hub touches.

|----------| 17cm 5.5 inches


|----| 10cm
|----| <-> | <---heres the mounting point sorry about average pic
|----|


|----------|

In other words if you divide the wheel width of 17cm / 2 = 8.5cm is centre to centre length and this is almost - 1cm from the centre of the wheel