It's at least closely competitive with all the other premium rear-drive compacts, but the Cadillac ATS isn't the leader in gas mileage.
It's a game of inches in this class. The ATS hasn't been officially rated as of yet, but according to GM's estimates, the base 2.5-liter four in the ATS will be rated at 22/32 mpg, with the turbocharged 2.0-liter four earning the same ratings. Both are estimated for the automatic transmission; no figures for the manual gearbox have been released, nor have any numbers for versions with all-wheel drive.
Cadillac says the V-6 ATS with automatic transmission will be rated at 18/29 mpg.
For contrast, the EPA publishes the 2012 BMW 328i's fuel economy at 23/34 mpg for the manual, 23/33 mpg for the automatic; the 335i is rated at 23/33 mpg with the automatic, 20/30 mpg with the manual.
The 3-Series benefits from stop/start technology that boosts mileage slightly, but even the ATS' other competitors fare a bit better in gas mileage. Audi's latest A4, with an eight-speed automatic, is rated at 24/31 mpg.
The ATS outpaces the C Class, at least, since it's rated at 21/31 mpg. And it far exceeds the lagging fuel economy of the Infiniti G37, which logged into the 2012 model year with ratings of 19/27 mpg and 17/25 mpg for automatic- and manual-equipped models, respectively.
Cadillac executives confirm that a diesel engine is a necessity for the ATS to be a success in European markets. They've hinted that the diesel could also become available in the U.S., but no official announcement has been made.