This is because in the combustion process (burning) if there is an oxygen molecule in the burning material that oxygen burns first (oxygen is required in the combustion process). Consequently this material with oxygen in it gives off less heat or energy in the combustion process. Of course all alcohols (added to gasoline to make gasohol) have an oxygen atom in its molecule.
Gasoline, made up of only hydrocarbons, has a higher energy content than an alcohol. This is why, according to the Energy Information Agency (a US government agency), states that Ethyl Alcohol (this is the kind of alcohol made from corn) has a heat content of over 83,000 BTUs/gallon, whereas gasoline has a heat content of over 124,000 BTUs/gallon.
This means when you mix alcohol with gasoline (as in gasohol) you get less miles per gallon. Actually you get 4 to 5% less mileage with a 10% mixture of alcohol in the gasoline to make gasohol. At 15% you get even less mileage. This means that if your car gets 30 miles to the gallon on pure gasoline with an alcohol mixture of 10% you will only get 28 to 29 miles per gallon. Hence you burn more fuel and use more fuel with this alcohol mixture.