Now that gas prices are hovering around $4 a gallon, many drivers are becoming more aware of their need to cut their gas costs. And with Earth Day upon us, we've been bombarded by messages telling us to make greener choices to help save our planet.

With all the talk we’ve heard lately about sustainable eco-friendly vehicles, I thought it might be useful to identify some pros and cons of the alternative engine choices available now and promised to come to market in the future.