Shika CNG vehicles are not as good for the environment as many seem to think. Methane is a far more potent GHG than CO2 and there will be plenty of methane leaking to the environment if we have tons of CNG vehicles running around. They are better for the immediate area they operate in though. So local effects are lessened global effects see little change. You also need to include the costs of having the Ram including lessened cargo and towing capacity in your calculations.