I found this tidbit but it looks dated :
There is considerable state-by-state variation in motorcycle registrations per population. The five states with the highest rates of registrations per 1,000 population in 2000 are Iowa (43.3), New Hampshire (39.4), Wyoming (38.8), South Dakota (38.7), and Vermont (35.7). The five states with the lowest registration rates are New York (5.7), Virginia (8.6), Texas (9.0), Maryland (9.3) and Arkansas (9.4).
While Alpine County (zip code 96120) and its central town of Markleeville is scarcely populated, so many locals ride motorcycles that it ranks as state's highest for the rate of registered bikes, according to a Record analysis of California Highway Patrol data from 2003.