Everyone pretty much knows the highway mileage rating on the 11 Hybrid is 23 versus the 20 on the 09. I have an 09 and just yesterday test drove an 11 Denali Hybrid. Noticed a couple of things, but the first is the 11 went into V4 mode a lot more on the highway than my 09. Noticeably more time in V4 mode at speeds of 60-75 mph. Also noticed the Denali had an operating back window instead of fixed, had a steel hood, bumper hitch cover, cooled seats, and I believe a heated steering wheel. Nice ride, and over its 87,800 miles had used 4550 gallons of gas, a nice 19.29 mpg average overall. Not bad.