225/65x17 = 28.515 diameter, or 89.582 circumference.
245/65x17 = 29.539 diameter, or 92.800 circumference.

Divide a mile, by the circumference to find the number of revolutions per mile. The 225 goes around 707.28 times,the 245 goes around 682.75 times.

The difference here is easily why you get a speedo reading of 60 when you are really going 50.