After all these years, Surtees remains the only one to win championships on 2 wheels and 4 wheels..
Well -
Not quite true if you're a follower of American racing. Joe Leonard was AMA # 1 (back in the days when only one rider could wear that plate, not like today when there are a half dozen different categories) and then had a successful career as an auto racer.
And Mike Hailwood, for one, gave it a pretty serious try, though an accident on the auto circuit put an end to a dual career.
As far as I know, nobody has begun in cars and switched successfully to professional level motorcycle racing. All you young MOrons, there's a challenge for you!