Spotted on the streets of New York (pictures)

u can't change the wheel size on the analog speedometer
if its calibrated to a 700c wheel, then thats alright, what if its not?

Change the dial face, print out a new set of numbers.

You would need some way of knowing you are going at a certain speed (perhaps borrow a 'real' digital speedometer?).

Get yourself up to 30mph (or whatever you can manage) check your speed on the digital speedo - wherever the hand/indicator is pointing on the analog speedo is - of course - 30mph, make a mental note.

Use this mark to interpolate back down to '0' when you knock up the new graphics for the new dial face.