Last time I heard, the dealers can't go larger than a 32" tire.
There are aftermarket solutions, and, its quite possible that an individual dealer invested in equipment that can perform additional calibrations.
An easy way to check for free, to see if you are calibrated...
Just hit the hwy, and look for those mile markers every 1/10 of a mile...
Go an indicated 60 mph on your speedometer, and, see if you go one mile according to the mile markers (NOT your odometer!) every 60 seconds.
If you are going a mile a minute at an indicated 60 mph...you're golden.
If it takes closer to a mere 55 seconds (Anything less than 60 seconds per mile...), well, you are NOT golden.
If for example the dealer calibrated for 32's, and you have 35's...you'd be going about 8.6% faster than the speedometer said you were, and about 8.6% further than the odometer said you were.
So - instead of 60 seconds to go a mile, you'd cover the mile 8.6% faster, or , in about 55 seconds.
Hope that helps!
PS - finding one of those unmanned speed trap sign things...or, using a GPS that gives your ACTUAL speed, and comparing it to what the speedometer says, would ALSO answer the question.
You'd need to be going at least an indicated 60 mph to be going fast enough to tell the difference with any reliability though.