Jeep Liberty switches from 6 cyl. to 4 cyl. at 50+ mph?

I went to a dealership and looked at the Liberty. The salesperson told me that the Liberty is not a gas guzzler and can get up to 27 mpg/hwy because the kicks into 4 cylinders on the highway if the speed is in excess of 50 mph. Is this true, or was this guy just trying to hustle me for his commission?