Volvo XC90 Coasting Transmission Deep Dive | How, when and why of coasting

Thu, Mar 25 2021

In our recent 2021 Volvo XC90 Recharge review, its turbocharged-supercharged-hybridized powertrain delivered impressive horsepower and fuel economy. But Volvo has one additional trick up its sleeve, propelling a car with power that's simpler, cheaper and all-natural: It's the power of momentum and gravity.

I've always been halfway to a hypermiler. I'm not obsessive about it, but in city driving, I enjoy timing stoplight approaches to keep the wheels rolling and avoid the inertia of restarting from a stop. There's little point to needlessly racing and braking between red lights, wasting kinetic energy (and therefore fuel). So I tend to drive strategically instead, often catching up with the drivers who jackrabbit but get hung up at the lights.

And, back when I owned a long line of vehicles with manual transmissions, I coasted.

Coasting used to be slightly controversial. Some claimed it doesn't actually save gas, though my mileage calculations showed otherwise. Another school of thought insisted that removing engine braking from the equation, even momentarily, constitutes a dangerous loss of control. Of course, an experienced driver can slip a manual transmission back into gear in a flash when engine braking's actually needed. And one should always use some common sense and judgment about when and where to coast. I'm not talking about careening down a 15% grade into a school zone.Â

Anyway, those arguments became moot when automatic transmissions pretty much took over.

(And no, never coast with a typical automatic transmission. Even if it weren't damaging to your type of automatic — but assume that it is Â— the risk of screwing up a nudge of the shifter from drive into neutral is too great.)

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But happily, some automakers in recent years have added a coasting feature to their automatics, with the aim of eking out more fuel efficiency. Volvo calls the feature on its Aisin eight-speed "Eco Coast." Some Mercedes, BMWs and others call it "sailing" or "gliding." The Hyundai Ioniq, Ford Mustang Mach-E and Polestar 2 are among EVs that allow you to cancel out all regeneration and freewheel downhill. And future cars such as the BMW iX are also being designed to do it.

By building coasting into the clockworks, automakers have taken any traffic safety concerns out of the question, because the car will instantly switch you back into gear when needed. And the fact transmission makers and automakers have built this feature settles the argument over whether coasting actually aids fuel economy. If it didn't, they wouldn't have bothered.

The XC90 Recharge, in addition to its electric and gasoline power, can coast when in Pure electric mode. The other powertrains, the T5 and T6 in the XC90 and other Volvo models, do it in Eco mode. Here's how it works:

First, switch to Eco, which, as economy modes go, is pretty tolerable for daily driving.

Once in Eco mode, your tachometer goes away and is replaced by the Eco gauge.

Then, you just have to look for opportunities. When you find yourself on a slope, or in a spot in traffic where you'd normally be rolling but not yet braking, lift your foot off the accelerator. You feel the engine load disappear, and the Eco gauge says COASTING, as shown in the image at the top of this page.

And in the words of Jim Lovell, you've just put Sir Isaac Newton in the driver's seat.

The gauge displays a green stripe between the diamond pointer, which indicates the car's calculation of best possible economical driving, and the long thin pointer, which indicates how you're actually doing at the moment Â— the greener the gauge, the greener you're driving. Shown below is how it looks when stop/start is activated. (A car at idle gets 0 mpg, so the diamond is at far right. But because the engine's shut off, the thin performance line is at far left, as green as you can get.)

There are caveats to the coasting feature. The transmission has to be in D, not in the manual shifting position. The car detects if you're on a downhill grade exceeding 6% and therefore won't initiate a coast. (That said, if you start out on a grade of less than 6% but the hill gets steeper, your glide will continue.) It will not initiate a coast if you're traveling less than 40 mph or over 87 mph. (But if you're coasting at over 40 and your speed falls below that, it'll continue the coast.) Finally, coasting won't work with cruise control.

The vehicle will maintain coasting until you touch the accelerator or brake, or move the shifter into the manual position. You may have to tap the brakes because you're closing on traffic ahead of you Â— remember, you're freewheeling while their engines are holding them back. When you remove your foot from the brake, the coast will not reinitiate. But once the way ahead is clear, a blip of the accelerator will get the glide going again.

A given coast might last for two seconds or several minutes, it just depends on conditions. The transition into and out of a coast is seamless Â— my passengers have never noticed I'm doing it.

I've glided along for miles at a time on a few long downhill stretches. Sometimes if the grade shallows out or you reach an uphill spot, you have to build up speed again before easing back into a coast, the so-called "pulse and glide" technique in hypermiler lingo. As long as you're gliding more than you're pulsing, you're probably seeing a net gain in fuel economy.

There's a section of I-405 right by my house where I can reliably coast for more than a mile while maintaining speed. That's a free mile. I drive it daily Â— so that's hundreds of free miles per year.

Regular use of coasting gains a couple of miles per gallon, at least according to the dashboard readout on my personal 2017 XC90 T5. The car's EPA highway rating is 25 mpg, but I typically get 28 to 30 mpg on road trips. I once logged 31.3 mpg on a drive to Sunrise at the 6,400-foot level in Mount Rainier National Park and back down the mountain to home again.

The car's EPA combined-mileage rating is 23 mpg. It's true to that when I don't coast. It gets 24-25 mpg when I do. Keep in mind there are fewer opportunities to coast on surface streets where you can't exceed the 40 mph threshold. Your biggest gains will be on the highway.

Now, a mile or two per gallon might sound like a minuscule payoff, but two points:

First, if you achieve even an extra 2 mpg, you've traveled 36 additional miles on an 18-gallon tankful. If that doesn't sound like much, try walking 36 miles and you'll gain some appreciation for the feat.Â

Second, coasting is fun. It's an involving way to drive. Seeing how long you can stretch a glide, and what numbers you can rack up, is as addictive as playing a video game. A lot of modern automotive tech has made driving boring; so has the slow death of the manual transmission. But here's one bit of technology that'll make you smile whenever it kicks in.

If you like buying gasoline and want to use as much of the stuff as you can, that's cool too. But if you find yourself owning a car that's designed to coast, roll with that.Â

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By Greg Rasa