[jbarham]

A cheap and easy way to increase the size of the car you already have is to buy a roof box. Our previous car was a small hatchback and interstate trips with our three kids in the back would have been impossible without the roof box. I only bought a bigger car when my oldest kid was pushing 6 feet and running out of legroom in the back seat of the hatchback.

[reassembled]

And increase drag, resulting in lower fuel economy/range.

[jbarham]

Sure, but I only used the roof box on occasional long trips when we had lots of luggage (e.g., visiting the grandparents interstate for Christmas). So I might have paid a few tens of dollars extra in fuel, but it was certainly far cheaper than buying a bigger car when we otherwise didn't need the extra space.

The roof rack and Thule box cost maybe $1200 AUD. But the rack was also useful for attaching bicycle carriers.

[kelnos]

A larger car is likely going to have worse fuel economy and/or range than a smaller car. If you get the larger car, you take that hit every single time you drive it. If you get the smaller car and a roof box, you take that hit only when you mount the roof box. If that's most of the time, then sure, it probably makes sense to get the larger car. But I don't think that's the GP's use case.

The larger car is likely more expensive to purchase in the first place than the smaller car plus the roof box.

[hr2016]

Meh, I'm currently on a cross-island road trip with. Family of four in a >10 year old, manual, brown diesel grandtourer/station wagon. Trunk and roof box fully loaded.

Long range fuel consumption at 5.3L/100kms, as usual at summer conditions.

Of course, ymmv : - )

[piva00]

Needs to be done in comparison with a larger car that would fit the same cargo as the smaller car with a roof box.