Main Motorcycle: Triumph Thruxton EFi
Join Date: Nov 2010
Location: Preston, North West UK
Other Motorcycle: Yamaha R1, Honda CB1000R
The figure before the 'x' is the width on the front wheel it looks like you only get one choice on width for each wheel diameter so the choice will probably be based more on required diameter than width (it would for me anyway as I'd be looking at 17" for my Thruxton), on the rear it depends on what width tyre you are expecting to run, I'd be looking to run as wide as possible (160 I think or possibly 170) so it would be the 550 rim.
Oh, the figure is in inches without a decimal point, put the point in 2 figures from the right and you have your size - 375 = 3.75"