Think yourself lucky you don't play a double bass or a harp...
1) Isn't not being able to run a Smart car an understood occupational hazard if you want to play tuba?
2) I can fit two EEb tubas in the back seat of my Clio - a pretty tiddly car. BBbs are more challenging, but go into any estate car if you fold a back seat down.
3) With most bands it is possible to leave the tuba at the bandhall between rehearsals anyway if you have storage problems at home.