Actually Marc, the 40/45 number is the ratio of the section width. Stock was 225/45, which is a 101.25mm tall sidewall, rear is 245/40 which is 98mm sidewall. While that is shorter, it's almost identical (the 3% difference isn't visible). Your new setup, 235/45, is a 105.75 sidewall.
A 40 series tire up front won't make a huge difference in shock absorbtion. I'd be very surprised if you could tell the difference, given the same brand/model tire. I also like having the same size all around to maximize tire life, btw.
I don't know if the MB GPS shows current speed or not. My handheld units will show current speed, top speed, average speed, and lots of other things. Check in the MB manual & see what is says!
