Quirks are anything that can reasonably give your Vehicle a once per Mission bonus by Pushing it or as part of Resisting a Consequence. "Uncontrollably Fast" an "Overpowered Boosters" are both Quirks that describe a Vehicle that is too fast for its own good, but the later is narrativized as a part on the Vehicle, though it doesn't take up any Load, just like all other Quirks.