It seems to me that, prior to this year, ethanol was added to the gasoline only during the winter months. However, someone told me that it is now in the gasoline supply permanently. Is this true? If not, when is it added and when does it come out?

It seems to me that, prior to this year, ethanol was added to the gasoline only during the winter months. However, someone told me that it is now in the gasoline supply permanently. Is this true? If not, when is it added and when does it come out?

Also, do you know if there have been any studies on ethanol negatively affecting gasoline mileage? It seems to me that my mileage has dropped about 10 percent over the last few months.

— Bill F., Medford

Wait a minute, Bill. You're asking about gasoline and it's not related to the price of a gallon? You must be an unusual fellow . . . or rich as the dickens.

At any rate, the Oregon Legislature passed a renewable fuel mandate in 2007 and it's being phased in around the state now, according to the Oregon Department of Agriculture.

The rule requiring blending gasoline with 10 percent ethanol went into effect in nine northeastern counties Jan. 15, in Jackson County and eight other Southern Oregon counties April 15 and will take effect in the rest of the state, mostly east of the Cascades, Sept. 16.

The federal Department of Energy reports that one gallon of gasoline generates about 115,000 Btu while ethanol generates 76,000 Btu. (A Btu, by the way, is a British thermal unit, a measure of the amount of energy required to heat a pound of water 1 degree Fahrenheit.) With less energy available, ethanol won't move your car as far.

Official projections from the state estimate that the E10 blend cuts into mileage about 3 percent. American Coalition for Ethanol, a trade group that promotes the fuel, estimates a drop of just 1.5 percent, which it claims is less than wind speed, stop-and-go driving and tire pressure. However, drivers anecdotally report drops of up to 10 percent.

The Since You Asked Used Car Lot doesn't have a certified mechanic, but we, and www.fueleconomy.gov, recommend making sure your engine is tuned up and using the proper grade of motor oil, your tires are properly inflated and air filters are clean.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by e-mail to youasked@mailtribune.com. We're sorry, but the volume of questions received prevents us from answering all of them.