Is coasting really such a clever idea?

Dear Bob: I drive a 2001 Honda Accord with an automatic transmission. I often drive on moderately hilly roads that have speed limits between 40 mph and 55 mph. When I drove a car with a manual transmission, I’d normally coast down hills to save gas.

With gas prices so high now, I’d still like to coast. But I’m wondering if it is really a good idea to put an automatic in neutral, coast and then put it back into drive at these speeds.

My dealer says that there is no point in doing this, because it will put a lot of wear and tear on the tranny and won’t bring the rpm down much, because the engine speed is computer-controlled.

My tachometer tells a different story, though: Engine speed goes down to about 800 rpm to 900 rpm, which at 45 mph is a decent gas savings. However, I do think the dealer may have a point about the wear and tear, so I’ve decided to do it only when I can coast for at least a quarter of a mile at a time. Unless you live in really flat country, you’d be surprised how often you can coast!

The question is, though, is it really a good idea to be doing this in a car with an automatic transmission?

— Sincerely, G.D., Niskayuna, N.Y.

A: The dealer does have a point — but there is a way to do this without taking the car out of gear, and it will give you even better fuel savings than you’ll get from coasting in neutral. Simply decelerate, with the car in gear and your foot off the gas. Believe it or not, you will get much better gas mileage by decelerating than you will by coasting.

While coasting may provide you with a quarter-mile or so of engine rpms in the 800-to-900 range, decelerating essentially shuts off the supply of fuel to the engine by creating a high-engine vacuum, the ideal condition for top fuel economy. Your tachometer may read the same amount of rpm, but when you’re decelerating you’ll use only a fraction of the fuel that you’d burn when idling at 800 rpm to 900 rpm.

So you can have your cake and eat it too. Decelerate down those hills! You’ll get even better gas mileage than you would by coasting, you won’t put any extra wear and tear on your transmission and brake system, and your vehicle will be in a safer traveling mode when it’s in gear instead of freewheeling, as it is when coasting.

Dear Bob: In regard to a recent e-mail about the loudness of turn signals: J.C. Whitney lists two types of louder turn signals on page 24 of its catalog. They cost $5.99 and $9.99. The more expensive one plays “Love Me Tender.”

Whitney can be reached at (800) 529-4486, or by fax at (800) 537-2700.

— Sincerely, W.R., Janesville, Wis.

A: Thanks for the info. Many of my readers will love you tender — or is it “tenderly?” — if these actually work as advertised!

Dear Bob: I’d like your opinion on the 2005 all-wheel-drive Mercedes 240.

I have a rear-wheel-drive Mercedes 220, which is not good in snow. Do you think there would be an improvement with AWD?

— G.R.P., Harrisburg, Pa.

A: All-wheel drive will make a vast difference in how your car handles snow, ice and wet roads. Compared to rear-wheel drive … well, there’s no comparison.

AMERICAN DRIVERS FIGHT BACK

Here’s a tip that will help you get better gas mileage while fighting terrorism and cutting our nation’s dependence on Mideastern oil:

Be sure your vehicle has the proper gas cap and that it fits snugly. Having the wrong cap or one that’s ill-fitting can cause poor performance, increase your vehicle’s emissions and reduce your fuel economy.

Most older cars use the vented variety, while newer cars with evaporative-emissions-control systems use a nonvented cap. Using an unvented cap on a vented system, in a worst-case scenario, could even cause the gas tank to cave in. A vented cap on an unvented system, on the other hand, can cause unnecessary pollution and wreak havoc under the hood, to the detriment of both engine performance and fuel economy.

Check your cap to make sure that the seal around the edge, where it meets the tank neck, isn’t cracked or frayed. If it is, replace the cap.

And when you finish pumping gas at the station, be sure to tighten the cap fully — a loose cap can cause as many problems as a bad one. On newer vehicles the cap will begin to click to indicate that it is fully tightened.