When Acura redesigned its popular MDX crossover for 2014, press materials specified 90.9 cubic feet of maximum cargo room, a significant increase over the prior MDX's 83.5 cubic feet. Still, something didn't quite add up. The new MDX was 2 inches longer from bumper to bumper, but height and width had decreased 1.5 and 1.3 inches, respectively. How had cargo room increased 8.9 percent?
Acura explained: Maximum cargo room was actually 68.4 cubic feet, not 90.9 cubic feet. It explained both numbers in the fine print on its consumer website as "based on [the] SAE J1100 cargo volume measurement standard plus floor space between first and second seats and front seats moved forward. This figure compares more accurately with most competitive measurements."
That should certainly clear things up for shoppers trying to find a car with enough room to get the kids to soccer practice or cross-country for a summer vacation.
What Acura doesn't say is that you can't even compare the 68.4 cubic feet in the 2014 model with the 2013 MDX's 83.5 cubic feet because Acura derived the numbers from different versions of SAE J1100. SAE J1100 is a widely adopted methodology from the Society of Automotive Engineers to measure interior space, but its roots go back more than five decades.
The Tower of Babel
Why is it so hard to compare cargo space? It's because different automakers follow different cargo space accounting methods. The methodology in question, called SAE J1100 Motor Vehicle Dimensions, is rooted in SAE drafting standards from 1963. A formal version was approved in 1973 with comprehensive revision in 1975. Eight subsequent versions came between 1984 and 2009, explained Neil Mitchell, a senior design engineer at GM. But there's little consistency as to which version automakers use. Toyota said it employs the 1975 version, which it says the EPA mandates for cargo volumes that classify a car as subcompact, compact and so on. Hyundai said it uses the 2002 version; Honda said it uses the 2005 version. Chrysler, GM and Ford said they use the 2009 version.
Six of the seven automakers we contacted use four different versions of the standard. The seventh automaker, Nissan, did not respond to our queries.
Honda told us in an email from its Ohio-based engineering group that the revisions change little.
The basic formula is the same: Take a cargo area and measure the length, width and height. Multiply it all together and that's your cargo room, explained Manda Mustaine, a vehicle-packaging specialist in Ford's engineering division.
But the versions of SAE J1100 alter which spaces in a car can be measured in the first place.
The Real World
"The inside of a vehicle is not uniform," Mustaine said. "You have wheel wells. You have a curved roof surface. It depends on where exactly you would measure." Most measures take an average between two lengths, two widths and two heights, she said. That accounts for the uneven nature of interior space, at least somewhat, because multiple measurements allow somewhat for, say, the curved tailgate or bulky wheel wells. But "there is potential that someone could take advantage of a potential nuance in the surface of the vehicle and say, 'Oh, no, no, we're allowed to measure this point here,' " Mustaine said.
Those dimensions can be manipulated by seat positioning, storage nooks and more. The Acura example illustrates this: The old MDX was measured using the 2002 version of J1100, said a former Honda representative who asked not to be named. (Acura is Honda's luxury division.) The redesign was measured using the 2009 version. If you measure the new MDX using the same method as its predecessor, its cargo room goes to roughly 75 cubic feet from 83.5 cubic feet, the spokesman said; measure it with the 2009 revision and the number falls to 68.4 cubic feet.
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We compared photos of the previous generation and current generation (above) with golf clubs behind the third row. You can see that even with the difference in measurements, there is little practical change to cargo space.
The redesigned 2013 Toyota RAV4 has a maximum cargo room of 73.4 cubic feet and 38.4 cubic feet with the rear seats in place, significant numbers in the class. When Jeep revealed specs of its all-new Cherokee SUV (which is longer and wider than the RAV4), the numbers did not look good in comparison at 54.9 cubic feet max and 24.8 cubic feet with the seats in place. Look at the side-by-side photo (below) with our grocery bag test (10 recyclable bags filled with identical items) and the extra 13.6 cubic feet behind the second row looks like it can accommodate only one or two extra bags in the Toyota. That's not a huge real-world advantage.
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What about seating position? Updates to J1100 through much of the 2000s attempted to clarify just where the seats go before you measure cargo space. Some used a robotic seat-measuring tool. Others reverted to SAE-spec mannequins with 95th percentile legs and 50th percentile torsos, Ford's Mustaine said.
The result created some consistency as to where the seats go before an automaker measures cargo room, but variances are possible.
"Can you influence things a little bit here and there? Yes," Mustaine said. "Just think of the seatback angle. So even if you use different [driving] positions, whatever you define as your seatback angle ... that can have more of a significant impact" on cargo room.
Trunk Versus Open Space
The type of cargo area matters, too. SAE stipulates a different method to measure space in an enclosed trunk in a sedan or coupe. Automakers have to fit SAE-specified blocks that simulate luggage parcels into the trunk and add them up. But the open spaces in SUVs, minivans, wagons and hatchbacks don't lend themselves easily to these blocks, and that's why the SAE and regulators invented the length/width/height calculations, GM's Mitchell explained.
That means you can't compare a sedan's trunk with a hatchback's cargo area and declare one larger than the other using only SAE cargo specs.
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Take the Ford Focus (above) as an example. It comes in sedan and hatchback form. The hatchback's cargo area behind the second row is rated at 23.8 cubic feet while the sedan's trunk is rated at 13.2 cubic feet. Looking at the images it's hard to see where the 10.6 added cubic feet would come into play in the grocery store parking lot. Stacking items above the top of the rear seat could be dangerous to passengers.
"In general, the enclosed-trunk method yields a result about 10 to 15 percent lower than the precise volume of the trunk because its purpose is to represent the usable cargo space for actual things owners will want to pack," Honda's engineers told us.
The Bottom Line
At the end of the day, automakers don't have to follow SAE guidelines when they publish cargo volumes. "If you look at the code of federal regulations, they state which procedure you're supposed to use when you look at [interior] volume," GM's Mitchell said. "But then you can measure it any way you want and use it in your advertising."
"SAE is not a governing body," Ford's Mustaine noted. "There is no advertising standard that is required by law."
All six automakers that responded to our questions said they follow SAE guidelines. Numbers can lie, so bring some salt and make sure you stack up our real-world cargo photos alongside those numbers when doing your research.
Editor's note: This post was updated on March 10 to reflect that SAE J1100 is rooted in 1963 drafting standards.
Cars.com photos by Evan Sears and Ian Merritt