2nd that

 

I'm confused? if you use 4.88 gears with 38" tires the drive ratios will be equal to stock resulting in no need to change any speedo gears or pcm mods.. Unless I'm missing something? its only when using tires or gears that are mismatched with each other that will cause your speedo to be off?

 

Changing from the stock tire size WILL change your speedometer. Taller tire = slow speedometer...shorter tire = fast speedometer.

Changing the gear ratio will only effect the rpm readings at any given speed. Larger gear ratio = increase rpm...smaller gear ratio = less rpm.

 

On the 98? and up trucks, vehicle speed is determined by the sensor on the rear diff, it reads a tone ring on the carrier. Gear changes will have zero effect, as the tone ring is still reading the same number of CARRIER revolutions. Tire size WILL have an effect, as you are changing distance traveled per carrier revolution.

 

Okay so I'm good then. Need to find a shop now.