actually i just saw your sig. you got 225/40/18.
this is right from
www.whalentire.com the place i got my rims and tires
SELECTED TIRE SIZE:225/40R18
DIAMETER: 25.1" SECTION WIDTH: 8.9" SIDEWALL HEIGHT: 3.5"
MINIMUM RIM WIDTH: 7.5" MAXIMUM RIM WIDTH: 9.0"
REVOLUTIONS PER MILE: 804
IF YOU TRAVELED 60 MPH WITH THIS TIRE INSTALLED ON YOUR VEHICLE
YOUR SPEEDOMETER WOULD READ 59.7 MPH
so your speedometer is almost right, it is .3 mph off.