The tax code already makes plenty of allowances for number of children and/ro dependents so I'm not sure why you think it'd be some herculean effort for a car size regulation to allow people with kids to have more cars. There's no reason for a childless couple to be driving a 7-seater SUV around everywhere. If they need a separate trailer for that one trip a year where they need to haul a kayak or something, that's fine, that still means they aren't burning all that extra energy the other 364 days a year.