To the original poster...$140 labor for 2 tires is standard (if there is such a thing) $70/hour shop charge. Which is not "bad" for piece of mind if you are comfortable with your dealer and trying to establish a good business relationship for future work.
As for tire loading, Hoople is correct when riding two-up in a dynamic riding environment, 2 or 3x loading can quickly eat up that margin pretty quick if you hit strong road drops and your suspension bottoms out, or when cornering and hit studder bumps or chuck holes. Adding trailer into the mix, I would trust Steve07...cause it is his business to know. JMHO
