Click and Clack's Cartalk

DEAR TOM AND RAY: Our gas mileage drops when my husband drives. He has a habit of driving with cruise control on all the time. He likes to engage cruise control as soon as he gets to 40 mph. But whenever we stop -- say, at a traffic light -- he'll slam on the accelerator to get to 40 quickly, then engage cruise. I think that's what's causing the falling mpg. I accelerate gradually, and we get better mileage when I drive. Am I correct in thinking his driving causes the loss, or am I nuts?

-- Lois

TOM: We have no idea if you're nuts, Lois. But we can tell you that you're correct about your mileage.

RAY: Absolutely. Hard acceleration kills your gas mileage. The engine has to work hardest whenever it's trying to increase the car's speed.

TOM: And when you make the car not only accelerate, but accelerate quickly, you magnify that effect.

RAY: When you accelerate hard, the transmission downshifts, or stays in its lower gears longer during acceleration. And because the engine runs faster, and less efficiently, in those lower gears, your gas mileage takes a hit.

TOM: Those readers who have "instant fuel economy" readouts on the dashboard will already know this. Your car may average 22 mpg, but when you drive away from a traffic light, even at moderate speed, you can watch your instant mpg reading drop to, like, 3! Then, as the car gets closer to its cruising speed, the mileage starts to go back up.

RAY: And since it's relatively easy for the engine to maintain a speed (rather than increase that speed), it can be done while remaining in a higher gear. So when you're just bopping along at a steady 40, 50 or 60 miles per hour, fuel economy is at its highest.

TOM: Keep in mind also, Lois, that your husband is not just lowering your mileage -- by accelerating hard like that, he's also shortening the life of the transmission, the engine mounts, the timing belt and most of the other "expensive" stuff.

RAY: So if you can't persuade him to accelerate more gently and cut this out, you may have to resort to some kind of aversion therapy.

TOM: Try this: Sometime when he slams his foot on the gas pedal, spill some hot coffee on his lap, and say, "Oh, sorry hon, it was the sudden jolt!"

DEAR TOM AND RAY: The owner's manual for my 2010 Honda CR-V with all-wheel drive states that only Honda fluids may be used in the car, except for motor oil. The dealer's service department confirms this. The rear differential gear oil, automatic-transmission fluid, power-steering fluid, brake fluid, engine coolant, etc., have to come from Honda, they say. Dire and catastrophic results are promised otherwise. I think "hellfire and brimstone" is in there somewhere, too. Is there a chance that Honda is overdramatizing this as a way to provide extra revenue to the dealer?

-- Stan

TOM: What the manufacturers tell us is that it's all about corrosion, Stan. Honda, and other carmakers, use different alloys of aluminum in their components -- mostly to make their vehicles lighter and more fuel-efficient.

RAY: And they say that their fluids are designed to minimize corrosion in those specific kinds of aluminum over time.

TOM: Is it possible that they also enjoy a little extra profit by selling the fluids? Sure. Is it possible that they benefit from having a lot of customers return to their dealerships for service so they can sell them other services and a 2015 Odyssey? Sure.

RAY: But they also may be absolutely right about their fluids minimizing corrosion on their cars. And it's in their long-term interest to not have key components of their cars corrode and fail as they get older.

TOM: Keep in mind that just because you want to use Honda fluids, you don't have to go to the Honda dealer for service. You can ask your independent shop to use Honda fluids, and it'll get the stuff from the dealer.

RAY: We have an independent shop, and -- with the exception of motor oil -- we actually use nothing but Honda fluids on all late-model Hondas.

TOM: So we don't have proof that using Honda fluids is absolutely necessary, Stan. But we think it's a reasonable thing to do based on Honda's claims. That's what we do for Honda owners who come into our shop.

Click & Clack (Tom and Ray Magliozzi) dispense advice about cars in Car Talk every Saturday. Email them by visiting

cartalk.com

HomeStyle on 06/07/2014

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