Drive it at what you think is 60, going by what the speedo says. Measure how many seconds it takes to go a mile (section lines or mile markers). Do it a few times and throw out the bad numbers and average the ones that are close to the same. Divide 3600 by the number of seconds. That will get you your actual MPH. Then divide actual by indicated to see what ratio of difference. You will need to decide if you need to go up or down in teeth. Since you are dealing with the driven gear, if you want the speedo to read faster, use less teeth. Pull the old gear, count the teeth, multiply or divide by your ratio you calculated and go buy a gear with close to that number of teeth. If you're lucky, you won't have to change the drive gear (tailshaft), only the driven gear. If it jumps around or doesn't move, you have the wrong gear match and will have to change the tailshaft or buy a gear box from your local speedometer repair shop.
Either that, or go to Canada and check to see if your truck is calibrated in KM/HR now!
