Lousy Mileage?
Is it your car, or your foot?
By Patrick Bedard

When the Gibson box arrives and you unpack your brand-new Lucille, just like the one B.B. King plays, here’s what won’t happen — you won’t find a tag attached saying, “Notice to consumer: Your sound may vary.” Damn right it’ll vary. No one with $2400 to spend on a guitar is silly enough to expect that curling his fingers around the neck of a Gibson Lucille will make him sound like B.B. King.

Your sound is you, baby. Get used to it.
And while you’re getting used to ideas, here’s another: Your gas mileage is you, too.
This last idea is resisted by some people who should know better. I’m thinking of Jeff Sabatini, who keystrokes on the topic of automobiles for the New York Times. In a piece last summer about the Lexus RX400h, he said, “How did it come to this, that Toyota is now selling a hybrid gas-electric vehicle with no tangible fuel economy benefits?”

His observation was based on round trips from Chicago to Grand Rapids, Michigan, in both the non-hybrid Lexus RX330 and the hybrid RX400h, casual trips apparently, three in all. The hybrid measured 20.9 and 23.0 on trips of 531 and 556 miles. The non-hybrid he credited with 21.6 in 462 miles.

We know nothing of Sabatini’s trips except that the miles between the same end points somehow came out different each time. We don’t know the passenger and cargo loads, or air-conditioning use, and most particularly, we have no idea how Sabatini drives.

“While this was not a controlled experiment,” he says, he nonetheless thinks we should accept his mpg numbers as facts, just as he apparently does when he flatly asserts that the ’06 RX400h “did not achieve better mileage than then 2005 RX330.”

Each car born into this world comes with two kinds of fuel economy — the EPA kind and the owner’s kind. The EPA tests are precisely done and highly repeatable. The owners’ results are as varying as their fingerprints.

To show that driver decisions matter, I ran a number of tests on Car and Driver magazine’s long-term Lexus RX400h. One series was designed to show the influence of speed on fuel consumption. This was done by repeating the same 13.7-mile stretch of interstate at constant speeds of 40, 50, 60, 70 and 80 miles per hour. Runs were made both ways to correct for wind and elevation changes. The onboard mileage computer was used to record miles per gallon. These were flying-start, flying-finish legs to eliminate the influence of acceleration and braking. I as up to speed when I pushed the reset button at a marker for the start; readings were taken on the fly as I passed a marker at the end.

The results are two-way averages, with only the driver onboard.
Consider:
40 mph -49.3 mpg
50 mph-41.0 mpg
60 mph-37.2 mpg
70 mph-31.7 mpg
80 mph-26.1 mpg

To see the effect of acceleration rates, I used a section of two-lane blacktop 0.72 mile long, with a wide spot at each end that would allow a U-turn at about 10 mph. Braking markers helped me maintain constant braking lap after lap. Ten laps were done with very gentle acceleration away from each U-turn, 10 with medium acceleration, and 10 with full-power acceleration. Once 40 mph was reached, speed was held constant to the braking point. Mileage was recorded on the trip computer.

Consider:
Very gentle acceleration-34.1 mpg
Medium acceleration -33.5 mpg
Full-power acceleration-27.5 mpg

To explore different braking rates, I used the same section of road, this time with constant acceleration up to 40 mph and three different braking points chosen to provide very gentle braking, gentle braking and average braking.

Consider:
Very gentle braking -34.6 mpg
Gentle braking -34.1 mpg
Average braking -32.6 mpg

Another set of 10 laps combined the best-mileage acceleration with the best-mileage braking to produce 35.6 mpg.

The tests above were all done with the air conditioning turned off. Repeating the best-best test with the air on lowered mileage by 1.2 mpg. I would expect this mileage loss to be greater on exceptionally hot days.

None of these results represents real-world driving. Instead, they are controlled experiments to show how driver choices influence fuel economy.

The driver is the wild card in any discussion of car performance. Those of us who came up through auto racing have learned to ask, “What does the car want from me to go faster?” Now I find myself asking, “What does a hybrid want for better mileage?”

So far, everything I see suggests that hybrids want the same things ordinary cars want. Power takes fuel. The less power you use, the less fuel you’ll burn. Higher speeds take more power, so fuel consumption rises along with the speedometer needle. Faster acceleration takes more power.

Hybrids have the magical trick of regeneration, which recaptures the kinetic energy of motion as charge into the battery. They regenerate to a small degree when you lift off the gas — what feels like the engine braking is actually regeneration in a hybrid — or to a larger degree when you brake. Regeneration is not perfect. There are losses. The electrical-to-chemical-back-to-electrical transformation, the round trip into and out of the battery, is only about 81 percent efficient, confirms Dave Hermance, Toyota’s U.S. executive engineer on hybrids.

What can the driver do about this loss? When you know there is a stop ahead, or a reason to slow, getting off the gas early — that offs the engine in Toyota hybrids — and recapturing the kinetic energy directly into the trip distance by coasting is surely more efficient than regenerative braking. Traffic behind may object to this technique, of course, and your own impatience may object even louder. You can also slow less and take the corner a bit faster. Driving for economy can be as thrilling as you want to make it.

Kinetic energy goes up with the square of speed, so 60 mph has four times the energy of 30 mph. But there’s a limit to how fast the battery will accept the charge; it can’t drink out of a fire hose. You can drive around this limit, Hermance says, by “shaping your stop.” Use lighter braking at speed, then increase it as speed drops to level the charge rate.

He offered another tip. Accelerate up to traffic speed, then lift off the power briefly. Often, the electric motor will take over when you resume, shutting off the gas burner. My first try, a 30.1-mpg trip into town, says the “Hermance lift” may be better than a perpetual tailwind.

Over 3394 everyday miles in the RX400h, I’ve logged 25.8 mpg. You don’t want to hear about my efforts on Lucille.

Article reprinted courtesy of Patrick Bedard and Car and Driver.

Photo by istock photo/Getty Images