[fw_fw]

I'm about to buy tires in a couple of months and am having the same questions. Comparing 225/45/18 to 225/50/18 shows a 3.3% increase in circumference, therefore when the 45 series shows 60mph the 50 series will actually be going 62.1mph. I checked my X1 against a handheld GPS today, and the speedo was reading 60 when I was actually going 58 according to the GPS. So 50 series will actually correct it almost perfectly. That and the Michelin Pilot Sport A/S 3s are 30 bucks cheaper in 50 series I've about decided to go with the 50 series tire.