5Dimes is head and shoulders the best book in the world for NASCAR. Not so great for Formula1 but have decent Indycar coverage too.
Other NASCAR options are
Bet365, TheGreek,
Betonline, Sportsbook. PaddyPower, William Hill and Betstars also have some coverage.
I like Betfair best for F1 but lots of books have good coverage and odds can be all over the place so worth having a range in your arsenal when the season starts.
Not into Speedway, Rally or MotoGP so not sure there.
Vroooom, Vroooom. Can't wait for car racing to begin again!