Only problem with that math is that the fenders / bumpers / rear tire stick past where the tire measurements go.
The full story is that I'm getting a new gooseneck. It has to be long enough to accommodate my TJ >AND< a JK. I can measure the TJ (~13ft), but don't have the JK yet. Need the bumper to bumper (plus spare tire) measurement so I get a trailer long enuf.