Car Advice

Continuously Variable Transmission (CVT) Explained

Continuously Variable Transmissions (CVT) are praised for being able to deliver a smooth driving experience and improve fuel efficiency, but with no fixed gear ratios how do they work?

IN A NUTSHELL, a continuously variable transmission, or CVT, doesn’t offer fixed gear ratios like a either a manual gearbox, automatic transmission or a DSG (Direct Sequential Gearbox, a newer type of auto). Rather, the CVT allows the vehicle’s engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds. The most obvious application for a CVT is when you’re trying to ensure maximum fuel efficiency, hence why Subaru has paired Boxer engines and all-wheel drive (all-wheel drive adds weight and can affect fuel consumption driving up the relative fuel consumption of a Subaru when compared with a non-AWD SUV in the same segment) with a CVT.

This article uses plain English to explain how a CVT works, why manufacturers are using them and how driving a car with one takes a little getting used to.

How an automatic gearbox works

A normal automatic gearbox uses gears to match engine speed to road speed. At low speed, the engine turns a small gear which is connected to a larger gear that turns the wheels via other transmission components. For every revolution of the small gear, the bigger gear only turns a fraction. It’s like using a low gear on your pushbike. And when you drive a car in first (lowest) gear, you’ll know that you can rev the engine quite high and the car doesn’t go very fast – this gearing is the reason.

At higher speed it’s the opposite. Now the engine turns a large gear connected to a small gear, so for every rotation of the larger gear the smaller gear turns multiple times. You’ll know from driving that in top gear your engine only needs to do say 3000rpm for 100km/h – whereas 3000rpm in first gear would see you only at about 25km/h.

In between the first and top gears are other gear ratios, and these days there’s usually a total of six but some newer automatics have as many as nine. The combinations of gears are carefully chosen so the engine can remain in its best rev range for either power, torque (turning force) or efficiency. But it’s always a compromise, even with nine speeds. And automakers also go to a lot of trouble to smooth out the gearchanges as well as make them faster.

So the standard automatic is somewhat compromised because the engine has to pick from only a few gear ratios. The reason this is less than ideal is because engines are best optimised to run only in a narrow rev range. The wider rev range the engine needs to work over, the more compromises the designers have to make. It’s like anything else that’s multi-purpose; you buy a camera lens that can do 18mm to 300mm and it’ll never be as good as a two lenses that just do 18 and 300mm respectively. There’s all sorts of technologies to help engines work over rev ranges such as variable valve timing, but nevertheless the fundamental problem remains.

Why is CVT better?

Enter CVT, or continuously variable transmission. This design does away with gears entirely. Instead of two gears, there’s two pulleys connected by a belt. The engine turns one pulley, and the other one is connected to the rest of transmission to the wheels. The magic of CVT is that the size of the pulleys can change from small to large, and infinite sizes in between.

So to start off with, the engine’s pulley will be small and the other one large, just like first gear. But as the car gathers speed, the engine’s pulley is smoothly reduced in size, exactly as the other pulley increases in size. In effect, you’re changing gear all the time. There’s a few different ways of achieving variation in pulley sizes, but the fundamental principle is always the same.

Continuously Variable Transmissions explained

What this means for the driver is that the engine can go directly to the best rev range for a given situation. This rev range is typically either the one that delivers the most efficiency, most power, or most torque. Once in its happy rev range, the engine just holds its revs and the gears “change” as the relative sizes of the two pulleys change. The result is a smoother, more efficient and quicker drive than an normal automatic. There is a downside though, and that’s the energy losses due to friction are greater with a CVT – about 5 to 15% depending on the type, whereas with normal gears it’s more like 2-5%. As ever, development is closing the gap, and the fact the CVT allows the engine to remain in its optimum rev changes more than compensates for the extra efficiency loss.

So everybody should be happy, right?

Nope.

The problem with CVT

CVTs sometimes feel awful; like a rubber band is stretching, and many drivers think there’s something wrong with the car. This is because when you drive off the engine will jump directly to a rev point and just stay there, while the car accelerates. This feels and sounds like the clutch is slipping in the car, or an automatic transmission gone wrong. It’s a bit like going out in a boat with a small outboard engine, you rev the outboard and the revs stay the same while the boat gathers speed. You also don’t get that immediacy of throttle control if you increase or decrease the revs, and that’s sometimes unpopular with drivers. In short, nobody likes the CVT experience, initially at least, because it jars horribly with everything they’re used to.

So manufacturers sidestep the problem by artificially creating “gears” in their CVTs. Simply, these are pre-set points where the designers decide that the two pulleys will be of certain relative sizes, just like normal gears. They then set the CVT to fix those pulley sizes, and when the engine runs out of revs it “changes gear” to the next set of relative pulley sizes.

Isn’t this just destroying the point of a CVT though?

Well, yes it is, but not entirely. Firstly, those “gears” are nothing more than software settings in a computer specifying the relative sizes of the two pulleys, not physically interlocking gears. So the “gear ratios” can be changed at any time, a bit easier to do than physically changing the lumps of metal called gears. Subaru have done this gear trick with the WRX, which in economy mode has six speeds, and in performance mode, eight speeds. It’s also how Toyota claim the CVT Corolla has seven speeds. It kind of does, but they could just as easily have put 16 speeds in there but nobody would take it seriously – it’s amazing, but CVTs are a good example of where marketing trumps good solid logical engineering.

Continuously Variable Transmissions Explained

Anyway, we need to examine why Subaru have chosen, apparently illogically, six speeds for efficiency and eight for performance. Why not eight for each? The answer lines within the heart of the CVT system. When in efficiency mode, the CVT in the WRX doesn’t precisely match an increase in revs to an increase in speed. You can see the vehicle change speed and the revs stay the same, within each of the six ratios. This is a small but important fuel efficiency measure. It’s not really noticeable unless you’re looking for it – because otherwise drivers would be upset – and of course it’d be better to have no gears and keep the engine in just one rev range, but we discussed why that’s not done.

In performance mode Subaru have locked the CVT out – any change in revs means a change in speed. The relative size of the pulleys changes only in the eight predefined ratios (“gears”). The effect is a more direct connection between throttle pedal and speed, and the sensation of revs rising exactly as speed rises – this is what performance and sports-minded drivers look for, although it’s ironic that the effect they desire is actually slower and less efficient.

Continuously Variable Transmissions explained

CVT transmissions also cannot handle as much power as conventional transmissions, although this is slowly changing. This is one reason you don’t see them on high-performance cars or large trucks. CVTs are often found in small cars and SUVs. Small industrial vehicles such as light tractors often use them, and you see them on snowmobiles too. The Aussie-built Tomcar small off-road vehicle (pictured above) also utilises a CVT, and Nissan’s new Pathfinder is one of the larger soft-roaders but it too has a CVT at its heart.

Summary – CVT pros and cons

A CVT car will be, on balance, more fuel efficient than a normal auto, particularly for stop/start traffic or when you’re constantly changing speed. It should be smoother and quieter too. However, the more of those advantages you get, the more the car feels motorboaty, where you get that strange sensation of increasing speed with the engine revs staying the same. If you can live with that, and really the only people who can’t are petrolheads, then a CVT could well be for you.


  • Chris

    Toyota have a heap of experience with CVT in their Prius hybrid models.

  • Kevin Moss

    Great article thank you. We are just about to buy a Mitsubishi Outlander with cvt and this article has really helped me get my head around why everyone keeps saying it takes some getting used to.

  • yogendra singh

    sir can anyone explain me how CVT good for vehicle than manual drive vehicles

    • Non Christian

      Smoother acceleration, better gas mileage, driver can pay more attention to the road. Less maintenance. No learning curve for new drivers.

    • nathan

      better fuel economic yes. but manual transmission will last forever, all you have to do is replace the clutch. CVT you have to replace the whole transmission when it is bad, there is no rebuild and it is expensive. it does not last long neither.

  • Ghazanfar A Shah

    This is an informative and well written article. How do the pulleys change diameters? or is it that the belt slips over to different diameter pulleys with the varying speeds.

    • bobbysgirl

      The gap between the sides of the pulley is increased or decreased according to the desired effect.
      Increasing the gap allows the belt to ride deeper in the “groove”, thus effectively decreasing the pulley size. Decreasing the gap has the opposite effect; the belt rides higher, as if in a larger pulley. The trick is, in order to maintain belt tension, the two pulleys have to act oppositely in unison. I guess only one pulley needs to be actively varied, while the other can utilize spring tension to close the gap thus maintaining tension.
      I used a drill press that worked on the same principle. No stopping the machine to move the belt to a different pulley… turning the hand wheel on top of the press allowed infinitely variable chuck r.p.m. while running. In fact, I believe you were not supposed to change the speed UNLESS the shaft was turning : )

      • Thanks Bobbysgirl you are correct. Sorry Ghazanfar didn’t see your comment until just now.

  • Barb Hardy

    After test driving 2017 Impreza a number of times I bought a $30000 car. I do most of my driving in residential and school areas with some start/stop peak hour traffic. By law these zones are 50, 40 and 60km however when travelling at these speeds the car jerks along. I drove test vehicle in my usual manner and no gear change problem. Dealership have had confirmation from Subaru that 2017 model doesn’t have the option of software upgrade to adjust settings. I’m told to just drive it harder….don’t suppose dealership will pay any speeding fines. I hate CVT. Can anyone offer possible solution please.

    • Hi Barb, I’ve tested the Impreza and haven’t experienced that issue. All I can think is its down to how you’re using the accelerator? CVTs don’t tend to like on, off driving which can see them jerk a bit, but then the same can be said of DSGs at low speeds. I’d need more information.
      And I’m not sure what the software upgrade you mention would be?
      Are you suggesting the car surges past the speed limit when you take your foot off the accelerator? Thanks Isaac

      • Barb Hardy

        Hi Isaac, the surge is only there at low speed (under 50km). When moving off I accelerate gently in one movement. I drive economically and treat my car with respect (I didn’t need a family car anymore so traded my 20yr old Camry V6 Grande, which was still in perfect condition). The Impreza feels like it’s missing, but I know that’s not possible. Surge/jerking also happens when I select reverse and I’m sure you agree I can’t reverse at speed as suggested by mechanic. With reference to the software update, he said on previous model the update would reset driving history so the car would adapt to any new driving style. Tuesday they contacted Subaru direct and they emailed that this feature isn’t available on new Impreza model. If I have to accelerate hard, doesn’t that defeat the whole economy element the CVT is designed for? Hope my reply isn’t too hard to desifer.
        Regards Barb

        • Subaru guy

          Not sure how many km’s you have done, but its not right. I suggest take it to a dealer, ask them to drive it. If they can drive it smoothly, then its your throttle inputs from your foot, But if they have same problem, then they may need to check it over.
          When you select reverse you should pause for a moment before pressing accelerator. I personally think its more to do with your driving than the car, as many people used to torque converter auto’s tend to think it should be changing gear at x point then lift slightly. As every manual car is different, i.e clutch slip/grab point, length of clutch “slack” Gear change feel etc, every Auto is different too, especially CVT autos.
          The only software update available is for infotainment systems only.

          Great article Robert.
          Plain English explanation!! Everyone should read this and try at least a few different CVT’s before saying all are rubbish.

  • Terry Fentress

    I recently acquired a 2005 Ford Freestyle from my brother. Its one owner, 192,400 miles but VERY WELL MAINTAINED with all service records. You are very correct is saying it feels like it is “slipping’….LOL. Gas mileage is nearly 20mpg in this AWD 3.0L V-6.

    My question….what is the life expectancy of a CVT ( I believe this is a Jaguar CVT)??

Robert Pepper

Robert Pepper

Robert Pepper is the editor of PM4x4, an offroad driver trainer and photographer interested in anything with wings, sails or wheels. He is the author of four books on offroading, and owns a modified Ford Ranger PX which he uses for offroad touring. His other car is a Toyota 86 which exists purely to drive in circles on racetracks. Visit his website: www.l2sfbc.com