According to this site:
http://www.fueleconomy.gov/feg/findacar.htm
1991 Mercedes-Benz 560SEL
Fuel Type - Premium
MPG (city) 14
MPG (hwy) 17
MPG (comb) 15
Annual Fuel Cost* $2421
1990 Mercedes-Benz 560SEL
Fuel Type Premium Premium
MPG (city) 13
MPG (hwy) 17
MPG (comb) 15
Annual Fuel Cost* $2421