I noticed a lot of people trying to keep the same circumference wheel and tire combo so the speedometer reads correctly. What needs to take place if this number is off?

 

It depends on what year we're talking about. On the older trucks, it'll take a different gear in the tail shaft on the transmission. On the newer ones with the first iteration of electronic speedos, it may take a different reluctor ring in the tail shaft of the transmission. On the newest, all it takes is a different scale factor in the PCM.

 

I'm no expert, but I would say speedometer recalibration. I think they sell a computer plug-in module at JCWhitney.com that can help you to recalibrate it yourself.

 

Like swart said. Plus I would like to add.
Depends on how much off were talking?
If your going to 33" or larger tires. You should also replace the ring and pinion gears in each axle too. If you dont, you WILL shorten the life span of your tranny.