We can use a ratio to solve this problem
P220/75R16 has a radius of 16
P250/85R17 has a radius of 17
The ratio of the radius of the tire to the speed is a constant
16 17
----- = ----------
60 x
where x is the speed on the speedometer
Using cross products
16x = 60*17
16x = 1020
x = 1020/16
63.75
63.75 mph
The larger the tire, the faster you will be going.