Easing off gas on incline doesn’t do much

I learned to drive on an automatic transmission. I’ve also driven manuals, but my aging shoulder precludes that, these days. Now I mostly drive a 2007 Prius with a continuously variable automatic transmission. The mpg is great, but when I go up hills, I have an old habit of lifting my foot off the gas pedal, as I did with older automatic transmissions, to allow the transmission to upshift and maybe save a little gas. I know I don’t need to do this with the Prius, but am I harming it in any way? Or am I simply wasting a bit of forward momentum and, therefore, gas?

— Paul

Dear Car Talk:

Well, I’ll tell you one thing you’re doing: By lifting off the gas on a hill, Paul, you’re giving other drivers

Answer:

even more reasons to hate being stuck behind Priuses.

People complain that Prius drivers are so obsessed with their gas mileage — to the exclusion of everything else — that they pull away from stoplights at an elderly snail’s pace, feathering the gas pedal, to get from 59.9 mpg to 60.0 mpg — and attain bragging rights at the sensible-shoe store.

This behavior tends to annoy the drivers stuck behind Priuses. Especially the guys in Porsches, who just spent 80 grand so they could speed from stoplight to stoplight.

I’m actually OK with you ticking off those guys.

The problem is that you’re not doing yourself or your car any good by lifting off the gas pedal on a hill. You’re giving up whatever momentum you had, and since it takes more energy to get a car up to speed than it does to keep a car at speed, you’re wasting gasoline by slowing down and then speeding up.

While lifting off the gas pedal on a flat surface may kick the Prius into electric mode, that won’t happen when you’re going up a hill.

You have a continually variable transmission (CVT) in the Prius. The way a CVT works is that it uses belts that travel up and down a set of cones to create an infinite number of gear ratios. Traditional automatics, like the ones on the cars you used to drive, have a fixed number of set gear ratios. So theoretically, with a traditional automatic, the ideal ratio for your car at a certain moment could be in between two of the fixed ratios.

And in that case, by lifting off the gas pedal, you might be able to force it to upshift — at least temporarily. This was a trick used by cheapskates the world over, including my late brother. And in all his years of driving, it might have saved him a 50th of a mile per gallon.

A CVT, on the other hand, will instantly, and constantly, find the ideal ratio for whatever your car needs at any moment. So there’s no need for — and no benefit to — trying to manipulate it while it’s operating with gasoline.

So drive gently, and try not to cause any more road rage than necessary, Paul.

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