Sizing from manufacturers varies slightly. Similar to shoes. But the same manufacturer of both bike and auto helmets will be the same as they use the same scale.
Honda Civic EX - Parkway Honda Company car
BMW 325I E30 Chumpcar Race Car
VW JETTA 1.8T WAGON Family Hauler (sold)
HONDA ODYSSEY 2nd Family Hauler (sold)
BMW 540iT M-Sport (New Family Hauler)