I upsized my tires from 245/75R16 to 285/75R16 and noticed a 5 mph difference in the speedometer. I used a Garmin GPS III+ to verify the speeds. I talked to 2 different dealers here in the Dayton, OH area and the 2nd dealer informed me about a VSS (Vehicle Speed Sensor) located at the rear of the transmission. The VSS spends the RPM data to the PCM (Powertrain Control Module) and the PCM sends, among other things, a signal to the speedometer.
What all this means is, change the gear on the end of the VSS and it will correct the speedometer error. Get a Haynes manual for your truck, it shows (Page 6-10) a good reference picture of the VSS.
Remember if your speedometer is reading low (Shows 50, going 55) get a gear with less teeth. My truck originally had a 20R (20 teeth R/H spiral) gear. 1 tooth = 5 mph. Since my speedometer was 5 mph low I got a 19R gear. The VSS is very easy to get to, 1 10mm nut holds it on a 1 electrical connector.
Also the "R" in 20R and 19R stands for R/H. When you remove the VSS the gear is held on by a small clip, remove the clip and place the gear on the table, shaft up. look at the teeth, if the end of the tooth at the top is to the RIGHT of the end of the same tooth at the bottom, you have an "R".
Hope this helps. The gear is a lot cheaper than a new set of tires. I got lucky, a mechanic at the dealer I went to had changed his tires and taken out a 19R, I got mine for free. I don't think the gear would be more than $20-30.

------------------
1997 F-250 Light Duty, Super Cab, Short Bed, 5.4L, 4x4, Gibson SuperTruck exhaust, K&N Filter, BFG AT(KO) 285/75R16, Westin Black Nerf Bars, SuperChip. West Milton (Dayton), OH

 

say scamp. where did you get the new gear for your VSS? can the dealer order one? did you have to take yours out first to determine how many teeth it had on it?

------------------
'97 F-150, 2WD, 8'bed, 4.2L,5-spd, 3.55 axle, Pendaliner, D. plate toolbox & mud flaps, K&N filter, 3" air box mod,slotted taillight covers, Ventvisors, bug deflector, Westin nerf bars, champion truck plugs, towing pkg, 2000# helper springs, Whelen Responder 2 light bar, 100Watt Street Thunder Siren and speaker, headlight flasher, Rancho RS5000 shocks, Heckethorn 2" leveling kit

 

I got the gear from a local dealer. You might want to call around first. The first dealer I called didn't know anything about the VSS at all! You will have to remove the gear to check the tooth count. According to my dealer most of them (but not all) have the number of teeth written/cast on the side of the gear just below the teeth. The removal the first time took about 5 minutes. the second time to remove, change the gear and reinstall took about 2 minutes.

------------------
1997 F-250 Light Duty, Super Cab, Short Bed, 5.4L, 4x4, 3.73LS, Gibson SuperTruck exhaust, K&N Filter, BFG AT(KO) 285/75R16, Westin Black Nerf Bars, SuperChip.