I just went through this problem myself.  On the way to Mountainfest I was riding with two other bikes (a Harley and a V-Star) and they kept falling further and further back.  I kept a close else on my speedo and I was going a steady 70 mph.  Finally my buddy pulled in range and flashed his lights to pull off at the next exit.

I was MAD because they couldn't even do the speed limit on those big bikes but before I got the chance to complain, they said they were doing 80+ at times to keep up and I just kept pulling away.  So when we got back on the Interstate each of them gave a thumbs up when they hit 70 ... I was reading 63!!!

No, still have the belt and still run stock IRCs.  It reads 6 or 8 mph less on the Interstate than what I'm actually running, but seems to be right on the money in the city.  It's like it gets worse the faster I go?!  Oh well, it's no big deal.  Someday I'll get bored and tinker with it, but for now I don't care.