The "green rating" is sort of an overall catchall, when you try to compare cars (new, in top condition) with each other.
Here are two vehicles compared:
2001 Subaru4 AWDAuto. Legacy Wagon-mpg 22-27, GGE 8, "Green" Sc. 6
2001 Subaru4 AWDMan. Legacy Wagon- mpg 21-28, GGE 8, Green Sc. 6
2001 Subaru6 AWDAuto. Legacy Wagon- mpg 20-27, GGE 8.3, Green Sc. 6
1998 Subaru4 AWDMan. Legacy Wagon- mpg 21-27, GGE 8.1, NA
1998 Subaru4 AWDAuto. Legacy Wagon- mpg 21-26, GGE 8.4, NA

2001 Jeep6 Gr. Cherokee 4WDAuto. SUV- mpg 16-20, GGE 10.8, Gr. Sc. 5
2003 Jeep6 Gr. Cherokee 4WDAuto. SUV- mpg 16-21, GGE 10.3 Gr R. 4-6
2003 Jeep4 Liberty 4WDAuto. SUV- mpg 20-24, GGE 8.8, Gr. Sc. 4
2003 Jeep6 Liberty 4WDAuto SUV- mpg 17-21, GGE 10, Gr. Sc. 4-6
2003 Jeep6 Liberty 4WDMan. SUV- mpg 16-22, GGE 10.3, Gr. Sc. 4-6

Comparisons for my own interest mainly, because I own these two. Note that the new Liberty, smaller than the Gr. Cherokee, is not better than the larger model.
NOTE that Greenhouse Gas Emissions are lower in the new Liberty 4 cyl., but the Green score is poor because of the other gases than CO2 emitted. The GGE number is not the only one that should be looked at. There are other gases to be considered.
The Subaru is a good vehicle, but a poor performer on the ranch properties here compared to the Gr. Cherokee. Like someone who recommended it, above, it is a good vehicle for general use. It won't haul my dog cages and gear, however; the Cherokee will.