I know this was an old topic, but I wanted to relay my experiences.
When I upgraded my stock Grabbers to Discount Tires Pathfinder (245/75/R16) I noticed my speedo and odo were off by about 10%. At 30MPH I was really do 33, 60 MPH, doing 66. Also my for every 1 mile traveled the odo registered .9 miles. Get it out on the highway and mesure between the mile markers, or better yet, watch a GPS while you're driving, both methods confirmed what my 10% reduction. When I adjust for the 10% my milage really didn't change much.
Hope this helps.
Andy