Some figures from the government. The "green" ratings are based on both mileage and emissions. CA= Calif. specs. Emissions
I looked up a few:
VW Passat                City 17    Hwy 24 green
VW Jetta wagon          city 24    Hwy 31 green 6-7
Subaru Forrester; Legacy    C 22    Hwy 27         6-7
Porsche Boxter             C 19    Hwy 27          6
Porsche Carrera             C 17    Hwy 25          6
Lincoln Town Car            C 18    Hwy 25         6
Lincoln Navigator 4WD SUV   C12    Hwy 16          1
Mercedes ML320             C 17    Hwy 21      1-4(CA) Bad Emissions
Mercedes E320 Wagon         C 20    Hwy 28       7
Lexus LX 470                C13    Hwy 16       0-2(CA)
Lexus RX 300 4WD          C18    HWY 22         6
Jeep Cherokee (disc.)       C 17    Hwy 22         4
Jeep Gr. Cherokee          C 16    Hwy 20         5
Ford Victoria               C 18    Hwy 25         6 often Taxi
Ford Taurus                C 20    Hwy 27         6
Ford Explorer (old model)   C 16    Hwy 19         4
Chrysler T+C AWD            C 17    Hwy 22         4
Chrysler Concorde          C 20    Hwy 28         6
Chev Suburban 2WD          C 14    Hwy 18       0-3(CA) Farmer's taxi
Chev Silverado 1/2T 1500    C 15    Hwy 22         4
Cadillac Deville            C 17    Hwy 27         6
BMW 525i                   C 19    Hwy 27         6

What the green ratings mean is a matter of debate. Didn't bother with the mini-cars.