Dear Car Talk:

I recently purchased a Kia Soul with an "Active ECO System" feature that can be turned on and off manually. The owner's manual states, "The Active ECO System helps improve fuel efficiency by controlling certain engine and transaxle system operating parameters." Why, then, is this vehicle not engineered so that this system is always active, to ensure better fuel efficiency? There must be some downside or tradeoff for turning on the Active ECO System, right?

-- Emmanuel

Yes: It turns your car into what is called "a dog." Notwithstanding the offense given to man's best friend, that's a term used when a car is sluggish or underpowered.

There's a direct correlation between how hard and how quickly you accelerate, and how much fuel you use.

So, what the Active ECO System does is it reduces the car's power and acceleration. It does that by lowering the shift points in the transmission (so it shifts at, say, 1,800 rpm instead of 2,300) and by reducing the amount of acceleration you get from stepping on the throttle.

I don't know how it does that in the Soul, but since all engine management is electronic these days, it can, for instance, change the parameters of the throttle position sensor and retard the engine timing. Basically, it makes the car a little less powerful and peppy.

And the reason they don't use those parameters permanently is that they're afraid people won't buy the car if they find it "underpowered" compared with other cars they test drive.

It sounds like you don't notice a difference, Emmanuel, when you engage the ECO setting. That means you're a gentle, responsible driver anyway. And since you were driving gently in the first place, you might not see much of a mileage increase in the ECO mode.

Similarly, if you drive like an animal and you engage the ECO setting, you're probably just going to stomp harder on the gas pedal to make up for the change in settings, and not see much benefit either. But if the Soul feels fine to you in the ECO mode, then just drive it that way. Why not? You may improve your mileage a bit.

And if you ever feel the need for a little more power -- if you're on a highway on-ramp, or driving a cluster of guides up Mount Kilimanjaro, you can turn off the ECO setting until you're back to your usual routine.