Re: Changing tire size and odometer/speedometer stagger
Whatever the percentage difference is in height, will be the difference in the speedo/odometer.
You can do things to counter this and read right. None are exactly 'cheap', but I use a ScangaugeII digital gauge/scan tool. It's one neat little gadget and I love it! It'll allow you to compensate for tire size, it'll do mpg's, trip cost, av. mpg's, miles driven, TONS of neat little things and it's even an OBDII scan tool. I actually gained about 2-3 mpg's by learning about fuel consumption by getting this tool. Check it out at Scangauge.com. *note: this will not change anything with your truck.....you look at this for speed, not your factory gauge.
Another option is heading over to the dealer to have them recal your PCM so that it'll read the single off the speed sensor correctly, and display on the dash correctly too.
But.....31's to 33's isn't that much of a difference. If anything, it'll be just remotely off. Ford's read too fast anyway so larger tires will counter that normal factory problem. I'd do the math for ya but I really don't feel like it.
'01 XLT disguised as a '03 Fx4 Level II.