If you had say, a "31/11.5-16" tire, "16" would denote the wheel diameter the tires is designed to fit, 11.5 the width and 31" the tire diameter.
I use 29" tires and my speedometer seems to function perfectly when compared to GPS mph readings. 31" tires would throw your speedo readings off a little, but I dunno if its worth the trouble of adjusting your speedometer.