The stock speedometer in your truck is calibrated by the manufacturer based on the stock wheel/tire size. When you place larger tires on your truck, a full revolution of the axle now makes your vehicle travel a farther distance resulting in an inaccurate speedometer readout.
How is that bad for me? Well if you like getting speeding tickets, then it's not. For the rest of us who prefer to not chit chat with the cops, it's very bad. A tire size increase of say 4 inches in diameter can result in up to a 15% change in your actual speed. When your speedometer is reading 60mph, you are going more like 67 to 70 mph and without knowing it became a prime target for any bored cop looking for something to do.
What can I do to fix it?Honestly, with all the variations in Makes and Models it's best to take it to a professional...unless you are really mechanically savvy. Newer trucks sometimes have adjustments that can be made in the computers or will automatically adjust because they only turn as fast as the axle turns so will recalibrate itself, older trucks may require you to replace the speedometer gear with one that correlates to your tire size. Get a manual for your vehicle and if it looks beyond your capabilities, get somebody who knows what they are doing.
Helpful ResourcesJeepin.com: Recalibrating Your SpeedometerSmiths/Jaeger Speedometer Repair ManualSpeedometer World 
