6% is what mine is off with 295's (33.4")
I pretty much make sure I'm driving the posted limit on my speedometer and figure I'll get away with the little extra. If you are in the habit of driving 5 over you'll get caught eventually with the extra your not seeing.
And yes the odometer will be off too, but in your favor.
Speski OffRoad LTD, Cargo Solutions for Toyota 4x4's http://www.SpeskiOffRoad.com
"80 series, with a fridge"
1976 FJ40, Project with little progress.
2004 Starcraft 10RT