10

Obviously running your engine at too high an RPM for too long isn't good. But what about running it too low? not so low that it stalls or lugs, but within around 1,000 RPM?

Would you get better gas mileage? Would it wear out the engine or transmission faster?

For example, going 20 mph in 5th gear down a quiet city street that you know has no stop signs for half a mile or more.

  • possible duplicate of Lugging the Engine (Manual Transmission) – Zaid Sep 16 '15 at 18:12
  • 4
    I don't think this relates to the Lugging the Engine question because a typical car won't be close to stalling at 1,000 RPM, that should be well above idle. – Poisson Fish Sep 16 '15 at 18:22
  • @Zaid edited to clarify. Lugging the Engine wasn't a duplicate, but the way I phrased it made it sound that way. – Jerreck Sep 16 '15 at 19:43
  • 1
    While I get that you're trying to understand a concept, this question still has too many variables. Please clarify a few things; Type or size/weight of the vehicle and what type of engine it has (gasoline/diesel) and how many cylinders. 1000 RPM is relatively fast for a diesel engine but rather slow for most motorcycle engines. – Nick G Sep 16 '15 at 19:52
  • 1
    @tomfumb, yes, you get more engine braking in lower gears, which is fine if you want to coast downhill at a certain speed or slow down in general. However, I don't think it would make much difference in an emergency braking situation. If you have ABS and slam on the brakes, your car is already going to be stopping optimally. If you don't have ABS, your tires might lock, which will make leaving it in gear moot anyway. Either way, if you are in a situation where engine braking makes the difference between hitting someone or not, you might be driving too fast for the conditions. – Poisson Fish Sep 17 '15 at 14:29
9

To answer your question, no it isn't bad for your car to keep it in as high a gear as you can while still maintaining speed. As long as you are above idle in RPM and your engine isn't lugging, you aren't doing any damage.

See this link for more information about lugging.

You mentioned that you might do this to be quieter and for fuel efficiency. Your car will certainly be quieter, so this is one advantage. As for fuel efficiency, there is such a thing as being in too high a gear in certain situations. Because of the vacuum created when the throttle is closed, and it will be close to closed at such a low RPM, you could potentially be making your engine work more to maintain speed than you would be if you were in a lower gear. This would mean you wouldn't be driving at optimum fuel efficiency. Of course, this depends on your car and many other factors, so there's no way to be sure being in 5th gear is worse for your fuel economy than being in 4th gear at such a low speed. If I had to guess, you will still be getting better fuel economy in 5th gear at 1,000 RPM.

When in doubt, to maximize fuel efficiency, use as high a gear as you can manage for the power required.

  • 3
    Speaking in generalities I agree with your answer. However, a slow turning engine also has a slow turning oil pump. This reduces the flow and potentially the pressure of the oil in the system. If there is not enough pressure to keep the hydrostatic bearings happy, you risk increased wear or potential bearing damage. – Nick G Sep 16 '15 at 20:08
  • 3
    @NickG - Oil pressure is not an issue. Every engine out there is designed to have enough oil pressure at idle to keep it lubed. At 1000rpm, oil should increase over idle, so better yet. You will not get additional wear or potential bearing damage due to a lack of oil, because there isn't a lack of oil. If the oil light comes on at idle due to lack of oil in the system or the engine is worn to the point of needing repair, then additional damage may occur ... this is not the fault of the low idle, though. – Pᴀᴜʟsᴛᴇʀ2 Sep 16 '15 at 20:38
  • @Paulster2 - I beg to differ. RPM is not the only variable when it comes to 'low' oil pressure. Engine load is also very key in determining if it's 'too low'. An idling engine requires very little pressure to keep oil between the bearings and journals because there is very little load on it. The pressure required to maintain the hydrostatic surface exponentially increases with engine load. The majority of spun bearings I've seen were related to too much load in too high of a gear. This is especially true in turbocharged applications where load can increase very quickly but RPM doesn't. – Nick G Sep 16 '15 at 20:57
  • @NickG - We aren't talking about changing engine load. We are talking about a static low load well above idle. There is more than enough oil pressure to handle this. If minimal oil pressure is considered 5psi on most cars (where the idiot light comes on), how is anything above idle going to cause an issue, even with a small amount of load. – Pᴀᴜʟsᴛᴇʀ2 Sep 16 '15 at 21:02
  • @Paulster2 - As I commented to the OP, there are too many unanswered variables. You're assuming that in this situation it is 'low load' and 'well above idle', but it may not be. Is this a truck hauling 20,000lbs, or a 250cc motorcycle? Gas or Diesel? It makes a huge difference. However, you can test what I'm saying fairly easily. Take a small vehicle with a 5/6 speed manual transmission and start off in 3rd or 4th gear, feathering the clutch enough so it doesn't stall. You'll hear some slight rod knocking as you start moving due to a lack of oil between the bearings/journals. – Nick G Sep 16 '15 at 21:17
5

tl dr - Yes to mileage; no to wear.

Would you get better gas mileage?

The general rule of thumb would be to run your vehicle in the highest gear possible without lugging the engine. This means if you can drive down the road at 1000rpm at 20mph, you will get better gas mileage than you will driving 20mph at 1500rpm. When you try to accelerate in the higher gear, this is where you will experience higher fuel consumption because you will need to be on the accelerator longer in order to obtain the higher speed. This is due to the lower torque available to the rear wheels. In a lower gear, relative torque is increased and you will gain your higher speed faster with the same (or less) amount of throttle input, which means you can upshift your transmission and get your RPMs down again.

Would it wear out the engine or transmission faster?

Just cruising at the lower speed in the higher gear will not incur any extra wear to engine or transmission. In fact, you'll see less wear due to running the engine at a lower speed. That difference will not be much of a difference, but there will be a difference.

If an engine is ran within tolerance (normal driving) for it's entire life, maintenance is kept up, and all else is normal, it stands to reason that there will only be so many revolutions an engine will make until it is worn out. If there is a set amount of revolutions an engine will make, running it at a lower rpm at the same vehicle speed will help it to last longer.

To reiterate something, though ... all that I wrote is dependent upon your premise of a flat road without stopping or accelerating. When you throw any hills, slowing, or speeding up into the mix, all bets are off. Downshift as needed so you aren't putting undue strain on your drive train.

  • 1
    "That difference will not be much of a difference, but there will be a difference. " This got me xD a difference indeed. – Jomar Sevillejo Feb 10 '16 at 0:06
3

The answer depends on the size of car and type of engine, but a modern car might have the tools to answer the question built-in. On my (base model, small, European, 6-year-old) car, just switch the dashboard display to show instantaneous fuel consumption and drive at constant speed on a quiet road. The current base-model version of the same car also comes with a gear-change indicator that suggests when changing up a gear will save fuel. I've only driven that model a few times, but it certainly suggested 5th gear instead of 4th at below 30 MPH in town driving. That gave about 1000-1200 RPM compared with the hot idle speed of about 850. (The logic includes the throttle position as well as the RPM - i.e. if you are trying to accelerate in the current gear, it is less likely to suggest you change up.)

Unless you try something unrealistic (like trying to drive up a 20% gradient in 5th gear at 30 mph) the engine management system stops RPM dropping below idle speed in any case, by increasing the fuel supply.

  • 1
    Actually, the engine management system increases the air supply and increasing the fuel supply is a corollary of increasing the air supply. However, this only applies to electronic throttle control systems. If the throttle is actuated by a cable, the separate idle air control valve that is controlled electronically is not big enough for anything else than idling. So, please keep in mind that the last paragraph applies only to modern engines! – juhist Feb 3 '17 at 14:58
1

In the last ~10 years, new engines have been designed to allow you to run them at lower rpm without lugging or hiccuping. The Volkswagen-Audi TFSI engines, for example, are built to run well at 1000 rpm in top gear. Older petrol engines I've driven needed closer to 1500 rpm to run smoothly.

  • Do the TFSI engines have variable valve timing? If not, this is very interesting, as the valve timing is probably not optimal for such low RPMs. – juhist Feb 3 '17 at 14:54
  • 1
    Yes, they have VVT. Also e.g. a twin-mass flywheel to improve behavior at low engine speeds. – Hobbes Feb 3 '17 at 14:57
  • I strongly suspect that is done predominantly because of the lack of understanding by most people about what gear you should be in under various circumstances.. – Bart Jan 21 '18 at 10:41
  • I don't think so. These features were part of a drive to lower fuel consumption, along with items like turbocharging, electric power steering and electric AC compressors. In the Netherlands, there's an information campaign to inform motorists they can now drive in higher gears than they've done for decades, suggesting people more often select too low a gear than too high. If you're in too high a gear for the engine, the signs are obvious enough even inattentive motorists know something's wrong. – Hobbes Jan 21 '18 at 11:29
0

This probably depends on what kind of engine you have, but engines having variable valve timing (VVT) technology can perfectly well run at very low RPMs. For example, I used to have a 2011 Toyota Yaris with 1.33 litre engine having VVT on both the intake and the exhaust. The engine could perfectly well propel the car at constant speed at idling RPMs on level ground if a low gear was selected. The idling RPMs vary from 500 RPM to 1000 RPM depending on the conditions.

For Toyota's simulated Atkinson cycle hybrid engines (that have VVT only on the intake), we know the engine operating lines which are computer-optimized to optimize fuel economy. The old 1.5 litre 1NZ-FXE had the operating line start from 1200 RPM and the new 1.8 litre 2ZR-FXE has the operating line start from 1000 RPM. For the bigger 2.5 litre 2AR-FXE engine the operating line is unknown, but presumably it is similar to the 2ZR-FXE. The hybrid vehicles using these engines (Prius etc.) run the engine most of the time at the lowest possible RPM. This is possible given that you don't demand too much torque from the engine at such low RPMs.

So, I would say that if your gas pedal is not more than halfway to the floor, you can perfectly well expect modern VVT engines to produce power at 1000 RPM, given that Toyota's hybrid eCVT runs the engine most of the time at this RPM. This applies even to engines having VVT only on the intake. However, please do keep in mind that if your power demands increase due to needing to accelerate, downshifting is advised. If driving a manual transmission car, double-declutch when downshifting!

  • The last sentence is incorrect. Double-declutching hasn't been necessary for any car made in the last 60 years or so, (barring a few cars where you needed to double-declutch going from second to first only). – Hobbes Feb 3 '17 at 17:55
  • Actually, double-declutching when properly done can save synchronizer wear and the shift feeling at your hand will be much smoother. Not necessary, true, but many cars have marginal synchronizers and it is downshifting that wears them. For upshifting, double declutching is not necessary at all unless skip-shifting. – juhist Aug 28 '17 at 13:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.