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The problem with this question is that it has been answered lots of times by people who don't know what they're talking about, so I thought I'd ask here in hopes that somebody understands the question I'm asking and not the question they feel like answering.

Here are some points to consider:

1) I don't care if it's illegal to drive downhill with no gear engaged, that's not the question.

2) I am not concerned with fuel economy in regards to this question, but I will say a few things on that topic, because I know somebody's just itching to tell me I'm wrong.

a) you use more gas in neutral coasting down hill because the engine uses gas to idle, but it does not use more gas because of the gas cutoff when engine braking above 1000-1500 rpm or so. I don't care.

b) Using your engine as a brake seems like a bad idea to me, it's extra wear and tear on the engine, which is much more expensive to replace than a set of brake pads. I'd rather replace the brake pads. And if you downshift to get some engine braking, and you don't double clutch absolutely perfectly, which nobody does, then you're adding even more wear to other expensive and hard to replace parts like the clutch plate and the sychros. Shifting solely to engine brake is a bad idea. No matter how much gas it saves.

c) And here's the kicker nobody ever considers: When you coast down a hill in neutral there's no engine braking, the car will increase its speed (let's disregard wind resistance and tire rolling friction for the moment). This means when you get to the bottom of the hill or start of the next hill you will have more momentum to get farther along it, than you would if you had been saving gas by engine braking. That loss of momentum, means you have to use MORE GAS to get to where you would be if you had just coasted in neutral. But nobody talks about that. And again, I don't care, I just want to know about transmission lubrication....

3) I recently bought a vw golf with an automatic transmission and in the manual it says don't coast down hill in neutral because the transmission is spinning a lot faster because of the wheels on the road than the engine is spinning to lubricate the transmission because it is idling. I wasn't aware that the engine spin is what causing the transmission to be lubricated, but if VW says that's how it is, then that's how it is.

What I was wondering is if the same is true in a manual transmission? (this is a bmw I'm asking about in particular) Can somebody who knows about the design of the lubrication system in a manual transmission explain if it is not going to be lubricated well enough if the engine is at idle or if the engine is disengaged because the clutch pedal is depressed?

While we're at it, they say the tranny fluid in a manual transmission lasts forever. I happen to know that nothing lasts forever, so when are you supposed to change the tranny fluid?

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  • I don't have a complete answer to your question, but to my understanding, manual transmissions (well, all gearboxes as far as I'm aware) are lubricated by literally being filled with oil. That is to say that there is no pump being driven by the engine to lubricate the transmission. I'm not sure why VW suggests that the transmission doesn't get lubricated as well when the engine is at idle, but either way I don't think it would apply in the general case.
    – Kitsunemimi
    Feb 12, 2019 at 21:10
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    Also, I don't want to get too off-topic, but just my personal opinion, I think the wear and tear you get on your engine when going down a hill is literally negligible compared to the wear on the brake pads. Like, even if all you do is drive up and down hills it may be economically worse to be braking. The reasons being: 1) you have no load on the engine, and 2) well-designed engines can handle operating up to redline for decently long periods of time, as long as they don't overheat.
    – Kitsunemimi
    Feb 12, 2019 at 21:15
  • @Kitsunemimi true, but brakes can be replaced again and again, over the 20 year life of a car. The idea that you shouldn't use brakes going down a hill is from the time when cars had drum brakes.
    – sam
    Feb 12, 2019 at 23:05
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    When you are coasting down a hill, need to do an emergency stop, and the brake pedal goes to the floor without doing anything, you might change your mind about "not caring" - assuming you survive the crash well enough to still drive, of course. There are a few hills near where I live, where the local bus companies have a compulsory "stop, then engage first gear and proceed" sign at the top - for good reasons. The hills also have sand traps for runaways - though the result can be amusing when someone tries to use a sand trap while the previous vehicle is still in it!
    – alephzero
    Feb 12, 2019 at 23:26
  • Manual transmissions don't use "fluid". They are usually full of oil that is much thicker than engine oil - e.g. 75W80, not 0W20. That stuff does "last for ever", in practice.
    – alephzero
    Feb 12, 2019 at 23:29

1 Answer 1

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If the transmission is in neutral and the clutch engaged, then if the input or primary shaft of the gearbox drives a pump the box will get lubrication, if the box relies on splash lubrication then it will also get lubrication.

However, if the clutch is dis-engaged ie you hold the clutch pedal down then the input shaft won't turn and there won't be direct lubrication... But, how long is the descent? If this is down mount Everest then lack of lubrication could be an issue, but for most hills the residual oil should be sufficient especially as there is little load on the gears.

Do note that the output shaft does not drive other shafts or gears unless a gear is selected...

Many manufacturers say the box oil does not need changing, but many users are finding changing it at 100k or 150k is beneficial.

The above is for manual transmissions, auto boxes rely on pumps driven by the engine and flow and pressure are dependent on engine speed, which is why VW are clear on that.

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  • bless you and your clear concise and helpful answer. And thanks.
    – stu
    Feb 12, 2019 at 22:31
  • This, of course, is why it's not advised to tow a car for any distance with the driven wheels on the ground
    – Nick C ♦
    Feb 13, 2019 at 9:31
  • There are things to consider which you do not mention. When the drive wheels are on the ground and are in motion, the output side of the transmission will be in motion. This should allow oil/fluid movement across the parts in motion, as long as the gears are splash style (as you suggest). If the clutch is depressed, with the vehicle in motion as described, all parts of the transmission normally moving (including any pump if so equipped) will be in motion ... they just won't be sync'd to the engine. If the transmission is in neutral while car is moving and clutch is depressed, that part ...
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 13, 2019 at 15:05
  • ... of the transmission will not be in motion, so won't be moving and causing wear. This should help back up what you're saying in your answer, so hope it is taken accordingly.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 13, 2019 at 15:06
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    @Pᴀᴜʟsᴛᴇʀ2 manual boxes I have worked on tend to have the first motion (primary) shaft always engaged with the second motion (layshaft) and the third motion (output) shaft is only connected to either the layshaft or the first motion shaft when a gear is selected. This means if the vehicle is towed and the driven wheels are rotating then with the box in neutral only the output shaft rotates, and, depending on the design and how high the vehicle is lifted there may or may not be any oil getting splashed around.
    – Solar Mike
    Feb 13, 2019 at 15:13

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