Butane, winter additives cut into mileage

Q You didn't mention (in a recent column) two other factors that cause lower fuel economy in winter.

Winter gasoline contains more butane and thus is less dense, meaning fewer pounds of gasoline per gallon. Since energy content of hydrocarbons (BTUs per pound) is fairly constant, this means less energy per gallon and lower mileage.

Do cold lubricants in the transmission, wheel bearings and differentials also affect mileage? How about colder tires; does rolling resistance increase? There has to be more resistance and drag in snow, right?

A The reason the percentage of butane, normally about 2 percent in summer blends, is higher in winter blends is to increase the volatility and vapor pressure of the gasoline so that it vaporizes more easily in cold temperatures. Remember, gasoline must be in vapor form when burned in your engine.

Interestingly, butane is also less expensive than gasoline, which may offer a small insight into why gasoline prices at the pump are a bit lower in winter.

And in addition to the higher percentage of butane, "winter blends" also contain additional additives such as anti-icing agents, which don't provide many BTUs of energy to burn and convert into power. Thus, there's less total energy per gallon of gasoline in your tank.

Yes, until lubricants warm up, thin out and flow more easily, and until tires warm up a bit to allow more flex and less resistance to rolling, it takes more power -- and thus more fuel -- to move your vehicle down the road.

My wife will be the main driver of this vehicle, and she'd like to know if she can leave it in 4WD Auto all the time when the roads are wet, snowy, or icy. I want to know if this will harm the vehicle in any way.

A "4WD Auto" is precisely what the name implies: a completely automatic four-wheel drive configuration. It is perfectly safe to operate the vehicle in this configuration full time, year-round -- not just in slippery conditions.

Doing so will not harm the drivetrain in any way. More importantly, it will automatically engage the four-wheel drive almost instantly if the drive wheels exhibit any slippage. This offers the driver much better traction and a much safer, more secure vehicle in all conditions.

Q We have a 1997 Dodge Grand Caravan with a 3.8-liter engine, front-wheel drive and 87,000 miles. The speedometer will periodically and randomly zip to 120 mph and back to zero several times, and then go back to normal. This seems always to happen when accelerating from a stop, at which point the car hesitates and jerks before regaining speed.

It also randomly happens while we're driving down the freeway (thankfully no jerking). It doesn't always flip between zero and 120; sometimes its behavior is more moderate -- between 20 and 60 mph.

We'd really like to get this fixed, but we're getting tired of dropping the car off at the dealer and having to share a car. Do you have resources that can help us?

A The diagnostic information in my Alldata automotive database -- which you can access at local public libraries -- outlines several possible causes for erratic speedometer operation. If there are no fault codes or problems with the powertrain control module (PCM), check the speed signal being fed to the PCM by the "distance speed sensor" used in the three-speed automatic transmission, or from the transmission control module used with the four-speed transmission. Then check the harness between the TCM and the PCM.