speedo question

[jcdillon110]

hey guys i got a 87 ranger my speedo is defentaly off due to the tires 33x9.50x15 how do i correct this? according to my gps i am about 6-8 mph faster than shown. any help would be greatly apperciated. sorry for the spelling I am half retarded

 

[Yellowsplash]

http://www.therangerstation.com/tech_library/Speedogear.html