Q Gary, why does diesel cost so much? In mid-2012, diesel was around 10 cents a gallon more than regular gas. Now it is about 40 to 50 cents more a gallon. Will it go back down soon?

Chuck Wilson

Morgan Hill

A I doubt it. Diesel was selling for a statewide average of $4.11 a gallon on Friday, while gas was going for $3.59 a gallon. That 52-cent difference has been the typical spread for several years.

Here's why, according to several analysts:

Going back to 2008, the global demand for diesel has increased, whether it has been for construction in China -- where oil demand rose 9.1 percent in November, the highest on record for the month -- or consumption in Europe, where there are many diesel vehicles.

In addition, diesel prices are always higher than gasoline during the winter in the U.S. because the heavy oil used to produce diesel is also used to produce home heating oil. The heavy use of heating oil in the Northeast drives up diesel prices in winter.

Also, we are now shifting to ultralow sulfur diesel, which, while cleaner-burning, costs refiners a dime or more extra to make, an added cost that is passed onto motorists and truckers.

Added Patrick-the-Energy-Man: "The United States has the only motor fuel economy where gasoline, not diesel, is the predominant fuel. With the increased usage of ethanol in gasoline and the decrease of overall gas consumption nationwide, the demand for gas is down. Diesel, because it is in more demand worldwide, can be exported. This tends to keep diesel prices higher. It also doesn't help that taxes on diesel are higher than gasoline.

"Prices for both products fluctuate, and sometimes diesel will be cheaper than gasoline, but my bet for the foreseeable future is that diesel will be more expensive than gas."

Q With diesel costing so much more than gas, does it make sense for me to reconsider buying a diesel car?

Tom Franklin

A Not necessarily. A recent study by Mellon University concluded that diesel-engine vehicles remain a better value than gas-powered vehicles because of lower operating costs and higher resale value over time.

The study, titled "Comparing Resale Prices and Total Cost of Ownership for Gasoline, Hybrid and Diesel Passenger Cars and Trucks," also found that the price differential between a clean diesel passenger car and a traditional gasoline-powered vehicle could be recouped in less than 18 months of driving. In addition, clean diesels deliver an average of 30 percent better fuel economy than their gasoline counterparts.

Q I have a Lexus RX which has consistently gotten 18 mpg around town and 22 mpg on the highway. I recently took a trip through Oregon and Washington and used their 92 octane premium. My highway mileage jumped to 27 mpg. When I returned to California gas I immediately dropped back to 22 mpg. Does the change from 91 to 92 octane make that much difference, or is it that combined with the effects of California's additives?

Peter Smyth

San Jose

A Mileage comparisons like this are hard to gauge. Gas experts tell me that your highway driving in Oregon and Washington likely had fewer starts and stops, more steady speeds and less braking -- all of which boost mileage, unlike on California's congested highways and streets.

Q For the last 15 or so years in California, there have been stickers on gas pumps stating that during winter gasoline would be oxygenated. Now I understand that all of the fuel sold here is 90 percent gas and 10 percent alcohol, so is there any difference in this state between the winter and summer blends?

Dennis Cole

Gilroy

A Yes. We have had oxygenated gasoline in California for a number of years, initially with MTBE, but now it is done with a 10 percent mix of ethanol.

But there also continues to be a summer and winter blend of gasoline. The main difference is the Reid vapor pressure, a measure of the volatility of gasoline. Low RVP gas is required in the summer, higher in the winter. Higher RVP makes starting easier in the cold, and low RVP evaporates (and pollutes) less in the summer.

Low RVP is harder and more expensive for refiners to make. To make the transition in the spring, tank inventories have to be low. This is one of the reasons prices run up every spring.

Go to Roadshow's expanded online presence at www.mercurynews.com/mr-roadshow and look for Gary Richards at Facebook.com/mr.roadshow or contact him at mrroadshow@mercurynews.com or 408-920-5335.