Why GM's SUV Mileage Dropped

CARS.COM — A revised catalytic converter, and the subsequent EPA fuel-economy tests that change required, is why gas mileage dropped on three popular GM SUVs, the automaker said Wednesday.

GM halted sales last Friday for some 60,000 three-row SUVs because the automaker had overstated fuel economy by 1 to 2 mpg in their EPA combined mileage ratings. Sales of the SUVs — the 2016 Chevrolet Traverse, GMC Acadia and Buick Enclave — will resume when GM prints and distributes corrected window stickers with lower fuel-economy ratings. Rather than the current 19 mpg in EPA combined ratings, GM said the new ratings will be 17 or 18 mpg, depending on driveline.

Related: Consumer Reports: GM’s MPG Discrepancy May Affect Older SUVs

All three SUVs, which GM builds off of one platform in Lansing, Mich., haven’t had significant mechanical changes for years, but they all carried similar EPA combined ratings: 19 mpg combined with front-wheel drive, and 18 or 19 mpg with all-wheel drive. Naturally, it begs some questions: What happened to make the mileage drop? And why were only the 2016 models affected?

As it turns out, GM actually did change something on the SUVs for 2016. It tweaked their emissions systems with a new catalytic converter.

Spokesman Tom Wilkinson told Cars.com the catalytic converter had “an updated design” and constitutes “a relatively minor change,” but he said any changes in the emissions systems require new emissions tests.

“Any time you change a piece of emissions equipment on the vehicles, you’ve got to retest,” Wilkinson said. “That retest just applies to the vehicles going forward.”

Fuel economy is a byproduct of those tests.

“When you run that emissions data, that spits out fuel-economy data with it,” said Nick Richards, a product development spokesman for GM. “And that data set was missed last year when they were putting together their numbers that go into calculating fuel economy.”

The automaker discovered the error when it was preparing 2017 model-year labels.

“The error was identified; we’re correcting it,” Richards said. “The tests and mislabeling does not apply to previous-model-year vehicles because it was a test on ’16 model-year vehicles.”

The updated catalytic converter will stay in the SUVs moving forward. Asked if GM is using the equipment for any other vehicles, Richards did not respond.

It’s unclear why GM needed a new catalytic converter for 2016, and why that change would impact mileage so much. Richards declined to say why the automaker made the switch.

Cars.com has tested the Traverse for mileage both before and after the catalytic converter update, and here’s what we observed:

• In early 2014, we took a front-wheel-drive 2014 Traverse 1LT on a 180-mile loop; this was before the catalytic-converter change. That test car’s window-sticker EPA ratings were 17/24/19 mpg city/highway/combined. Using the SUV’s trip computer, we got 19.1 mpg.
• In September 2015, we took an all-wheel-drive 2016 Traverse LTZ on a 136.5-mile loop. That model year had the updated catalytic converter, and its now-updated EPA fuel economy is 15/22/17 mpg. Still, we got 20.2 mpg using the trip computer.

Those were two tests in different climate and geographic conditions, so the results can’t be directly compared. Despite their different drivelines, both SUVs carried the same 19 mpg EPA combined rating before GM revised the 2016 numbers. We followed rigorous mileage procedures for both — rotating drivers (seven in the 2014 test, five in the 2015 test) throughout the drive loop, avoiding cruise control, keeping windows and sunroofs closed, and using air conditioning. Both routes involved a mix of city and highway mileage.

An EPA spokeswoman told us the agency’s engineers would look into GM’s emissions update.