So the 1993 GMC Crew is on the road (finally!), and the speedo seems to be off as expected. At 50mph on the autolite speedo, the GPS reads about 44mph. I'm running a dodge nv4500 and a gear driven speedo. I've found a web site that lists all of gears that dodge offers, but then i realized that I've never been through swapping speedo gears to know the formula, or which gear would be right. Can anyone share some tips and tricks on getting a mechanical speedometer to read the correct speed, or is it more of a trial and error experience with the different toothed gears?