They can be run on less than premium, but they shouldn't be. If you do a search, you will find lots of informative info on this matter.
I was not able to find mph data from MBUSA, but I know that my 1995 C280 (Manufacturer's Data) gets 20 city and 26 hwy. I am always flying all over with always a mix of city and hwy and never get below 19 mpg.
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)