There may be another way the dealer doesn't know about..
On third gen Rams you had something like 3 different tire sizes to choose from but a little known method of solving for odd size tires (like I have) is to go through the anti-lock brake screens using either the dealers Star-scan tool or an after market programmer like the Smarty Jr. Once you get into that section you can adjust tire height in .1" increments and get your speedometer dead on.
If you go this route, measure the height of your tires and subtract about .5" to get the closest accuracy without having it read too slow.
