The OP didn't give any info on his truck, so we can't answer how to fix his speedo calibration. If he has a 99 or newer F-150, he can recalibrate with one of the aftermarket computer tuners.
In theory, you could recalibrate the speedo by changing the gears in the differentials but that would depend on what gears are available. The gears he needs may not be available for his truck.
i had 255/70/16's on my truck and put 295/75/16's and at 70 mph its about 11mph off....yes ELEVEN.....if i put cruise on 64 its actually doing 75.....my Garmin GPS says about 77 but its a little off i think
people that say its off by X mph are not giving accurate description.
its would be off by a percentage. If your 31" tires are factory, then the 33s would be 93% of the 31s. so if you are actually doing 20, the display would show 20x.93=18.6 If you're doing 75 the gauge would read 75x.93=69.75. So at 20 its off by 1mph, and at 75 its off by 5.
Yeah its definitely going to increase with speed but at the only place it really matters, (highway), mine is off by atleast ten.... around town doing 30-45 its off anywhere from 4-6mph but thats not a huge deal cuz you dont really look at ur speedo to much around town anyway