I have recently mod'd to a 19 front. calculating the ratio it was the same as a 44.8 rear sprocket.('07 sm 18/42). I have noticed a little drag when trying to race off in 1st but nothing that will cause detriment. 70mph in 5th @ 4400rpm, stock gearing, with new on 70mph in 5th @ 3800rpm. Feels much better for riding on, I've had it out on a variety of roads and it's good. I've run it up to 100mph and not touched 5500rpm, by my calcultions it should be good for 135mph (ish).
The speedometer has to overread according to manufacturing standards, but the maximum inaccuracy allowed is <10% changing to a 120/80 tyre is an increase in wheel circumference of approx 10%, taking up the inaccuracy. The tyre shape should not be so different as to affect the handling (contact patch). Unfortunately manufacturers now are not necessarily using components which are best for the vehicle but instead allow them to pass the vehicle through manufacturer required standards. A prime example of these sacrifices are for emissions and noise requirements. (This is my current project at work.)
