The only time you'd need to change out the speedometer gears is if you source a 4-speed from a big truck which would have a higher numerical rear end ratio than a 1/2 ton. Stock ratio for 1/2 tons is 4.11 and a bigger truck would have a 4.56 or higher gear ratio.
Here and on the following page is information of stock speedometer gear sets based on truck and tire size and rear gear ratio.
