A more realistic, real numbers based discussion:
Well-to-Wheel Energy Use and Greenhouse Gas Emissions of Advanced Fuel/Vehicle Systems – North American Analysis by Center for Transportation Research, Argonne National Laboratory, June 2001.
Energy Efficiency of Gasoline
For conventional gasoline, the study found that it takes about 241,000 Btu to produce and deliver 1 million Btu to a gas tank in the form of gasoline (50% probability values are reported here). The efficiency calculation is 1,000,000/(1,000,000 + 241,000) or about 81%.
To deliver 1 million Btu in the form of gasoline takes 241,000 Btu for recovering crude oil from the well, transporting the crude to a refinery, refining crude oil to gasoline and finally transporting the gasoline to a service station. The energy expended in exploration for crude oil is not included in this calculation.
In other words, each Btu of gasoline energy requires about 1.2 Btu of energy input, which includes the energy contained in crude oil plus the energy consumed in converting crude to gasoline.
Energy Efficiency of Ethanol Produced from Corn
For ethanol produced at a dry-mill plant, with co-product credits calculated on a "displacement" basis, corn-based ethanol efficiency is 63%.
An adjustment for "co-product credits" is necessary because corn-based ethanol plants produce other products in addition to ethanol. Distillers´ grains and solubles (DGS) is a CO-product of dry-mill corn ethanol production. DGS is used in animal feed. To properly account for the energy used to produce fuel, the total energy used at a dry mill ethanol plant must be allocated between the fuel produced and the co-products. Two methods of allocating the energy inputs are the "displacement" method and the "market value" method. The displacement method is the more conservative approach (lower credit given). The displacement method starts by estimating the amount of CO-products produced. Second, the products to be displaced in the marketplace by the CO-products are identified. Third, the displacement ratios between CO-products and the displaced products are determined. Finally, an estimate is made of the energy that would be needed to product displaced products. This estimated amount of energy represents the "energy credit" of the CO-products
The energy efficiency of corn ethanol as described for this pathway is 63% (based on 50% probability values). It takes about 587,000 Btu in total energy to produce and deliver 1 million Btu of ethanol fuel to a vehicle fuel tank.
According to this study, it takes more energy to produce and deliver ethanol to a fuel tank than it does to produce and deliver gasoline. Every Btu of energy in ethanol requires about 1.6 Btu of energy input, which includes the energy consumed in the production and transportation of fertilizer and other agri-chemicals, farming corn, transportation of corn to the ethanol production facility, production of ethanol and transportation of the fuel to a fueling station.
However, there is an important difference between gasoline and ethanol motor fuels. The Btus in corn or other biomass feedstock are renewable, unlike the Btus in crude oil. The calculation of efficiency based on total energy input is therefore less meaningful for renewable source-based fuels. A better indicator of the energy balance for renewable biofuels is the ratio between the energy content of the fuel and the fossil energy used for production. If the public is concerned about reducing the use of fossil fuel (and reducing the associated emissions of carbon dioxide into the atmosphere), then the focus should be on how much fossil energy is consumed.
When you burn gasoline in your car you are consuming the fossil energy required to produce and distribute the gasoline plus the fossil energy contained in the crude oil from which the gasoline was made. Production and distribution of corn-based ethanol requires more fossil energy than production and distribution of gasoline. However, when you burn a gallon of ethanol in your car, your consumption of fossil energy is only the fossil energy used in production and distribution. The feedstock is renewable biomass, not crude oil.
By considering only the fossil energy inputs, the energy efficiency of corn-based ethanol is 170%. Every Btu of energy in ethanol fuel consumes about 0.6 Btu of fossil energy. In comparison, every Btu of energy in gasoline consumes about 1.2 Btu of fossil energy.
With renewable biofuels, you get more fuel energy in your tank than the amount of fossil energy used to get it there. This calculation provides some indication of the enhancement effect of renewable fuel production in helping to stretch out the use of limited nonrenewable resources.