I bet the computer or ECU is seeing a different ratio now between the distance the rear wheel travels in rpms compared to the different larger front wheel on the bike now. I would tend to believe it sees the rear of the bike as going faster than the front now, since the front wheel now does NOT turn as many rpms in a given distance as it use to. So it is possible they may just reprogram it for a 21" front wheel or it may be even the possibility of having to change out the ring to a different size. A good dealer ought to be able to tell you or a service rep. I would be willing to bet you have to put the program in for the Magnum is my guess.
Buckeye, Arizona(USA) 2011 Victory Cross Roads, Black police bag guards, black engine guards, black handle bars, Triumph Tiger mirrors, Triumph hand grips, set speedo/tach back 5" , adjustable timing wheel, Lloyzz Fuel Programmer III & air filter. Modified exhaust, made stock into 2 into 1. MRA windshield, Rotated front fender forward,relocated ignition switch, Bad Ace Fuel Tank Risers, Fat front tire.