Due to our sue-happy culture, most factory speedos read a little fast anyway (if you found you were going 1mph faster than the speedo said it would be "unsafe", and you could sue the car maker, but the other way around is "safe"), so going to a rear tire 2% larger may actually CORRECT your speedo. That is exactly what I found when I changed the tires/wheels on my S2000. The slightly larger rear tires just about exactly corrected the speedo. I tested it using GPS before and after.
09 G8 GT, Liquid Red, Premium, Sport, Sunroof, Two tone