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 — otherwise known as accelerating.
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, and acceleration diminishes, 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, your fuel economy is at its highest. That's why, except for with a few hybrids, highway mileage is always higher than city mileage.
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 of 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!"
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)