Here's my $.02...
It should be no problem to upgrade. Any good tire shop will do the plus sizing for you. Simply tell them the stock wheel and tire size. Assuming you're starting with a 14" rim simply ask for a plus one (15") or plus two (16") etc.
Make sure you get the correct bolt pattern and offset for the wheels.
If you correctly upsize, the speedometer inaccuracy would be minimal.
Cheers
