HansO be correct.
FYI - if not too wide, that 245/50 would be .3 inch smaller circumference that the 225/50 and just .6 inch larger circumference that the OEM motorcycle tire.
Bear in mind that these calcs are also affected by mounting on the motorcycle rim width versus the intended car rim with.
This got me interested, so I checked and found that Riken Raptor is recommended for a minimum rim width of 7 inches. Since the Rocket Roadster rear wheel width is 7.5 inches, I would think it be no problema in the mounting department and have little affect on the circumference calcs.