Any evaluation of alternate fuels and their contribution to climate change must include consideration of "energy density", a measure of the amount of energy stored in a given volume or mass (weight) of that fuel. The chart below shows the energy density for some common fuels ranked by volume. The units of measure for energy density are Mega Joules, mass is expressed in kilograms and volume in liters. Uranium contains far more energy than any of our other fuels, and behind nuclear fusion of the sun is the second most powerful source of energy known to man. While nuclear power is does not produce greenhouse gases, it does have other environmental liabilities, and is not yet suited for mobile applications, such as vehicles.

Values abstracted from Wikipedia

A comparison of the energy densities of heating oil and natural gas will readily show why it must be piped in, rather than stored on site, as a 1,000 units of natural gas are required to obtain the same amount of energy as one unit of heating oil.

Per gallon, diesel fuel contains about 12% more energy than gasoline, thus when burned in engines of comparable efficiency means that diesel powered vehicles should get about 12% better mileage, which is in the ball park.

As both gasohol and ethanol have far lower energy densities than gasoline, they will accomplish less work per unit of volume. In fact, a car burning one unit of ethanol will only travel 70% as far as the same vehicle burning the same volume of gasoline. The only valid standard of comparison when considering the production of pollutants or greenhouse gases is the amount of work actually accomplished (miles traveled), not the amount of emissions per gallon. This basic fact is often over looked, or misrepresented, in discussions of alternate fuels.