It confuses me too. I am thinking they take the opening size of the door (for the minivans) and then measure that back into the car until they stop (wether at the third row, second row...wherever) so any slopes in the roof or indents on the side aren't accounted for in the total space? But the SUVs it is actual space you can use? I don't know...we need Darren's input...he is good at this stuff.
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Most measurements you find online all come from the manufacturers, based on some standard method of measuring cargo space. Some manufacturers go farther and also measure behind third row, behind second row, etc. Hopefully, these are comparable from one brand to another, but I don't really know what the standard is or how they measure it. Shopping sites like Edmunds and KBB and Cars.com usually just use the manufacturer numbers, so most will have the same ones. Some may do their own testing and if so, should somewhere say how they measure.
The leading consumer magazine I mentioned typically does all their own measurements in some way that makes sense to them in a practical nature that would apply to more real-world use. Obviously, their real-world use may not be the same as anyone else's, whether it's cars, carseats or whatever. The cargo measurement is done the exact same way for all minivans, suvs and wagons. They have another method for sedans, hatchbacks, coupes, etc). I didn't quote the entire pop-up window as I had to type it manually since it wouldn't let me cut/paste. I think the opening size and door is a consideration from the text, but that is obviously important if you are carrying large items like dog crates or whatever. Their method would also seem to lose a lot of the "nooks and crannies" that may or may not be useful.
So, despite the numbers from two sources, it may well be that some full size SUVs have as much or even more usable cargo space than minivans, or maybe it varies a lot depending what you are carrying. I think the passenger "box" length of most minivans and SUVs is pretty similar, since the longer SUVs use up a lot of that extra length for the engine compartment. Perhaps we need someone with a new Sienna and newer Suburban to have a cargo showdown?
One thing I know for certain is that no midsize SUV is remotely close to Odyssey or Sienna (and presumably other minivans) in terms of total cargo space, cargo space behind the third row and 3rd row comfort/legroom. You typically pay more for a midsize crossover SUV and generally get less than a similarly equipped minivan. I've reviewed a lot of the popular midsize crossover models in the last 1-2 years and even larger ones like the Enclave fall well short of a minivan in terms of space. You pay even more for a truck-based, full-size SUV, but at least those that actually need the heavy duty towing and rugged trail offroad capability can get similar (or greater) passenger and cargo space to a minivan (your mileage may vary, of course).
Attached is a shot from a recent trip in our Toyota Highlander midsize SUV. All that stuff on the hitch carrier would easily have fit behind the third row in our 2006 Odyssey, along with the suitcases behind the third row in the Highlander. Granted, we averaged about 31 mpg on the highway over 1250 miles with a full load of people, stuff and A/C use. I *once* almost got to 30mpg on a tank under ideal highway conditions with minimal A/C use in the Odyssey, but usually more like 26-27 highway. Not a real fair comparison since it's a hybrid, but I'm happy that I didn't lose fuel economy by downsizing;-)