In mainland Europe the rule is actually occupy the free rightmost lane unless when overtaking.
In other words, the leftmost lane should be used exclusively for overtaking and not to cruise in.
Indeed doing so is an offence and could result in a fine regardless of whether you are driving at the speed limit
and nobody can legally overtake you.
It follows that one should never undertake (overtake on the right) ( although some countries do allow "passing" on the right ).
The Wikipedia page on overtaking is very informative on the subject, and has a list of rules regarding undertaking by country.
travel.stackexchange.com/users/22140/JoErNanO
In general, no .
Germany e.g. changed the law from the necessity to drive right at all times to allow staying in a lane if and only if there are vehicles on the right side for approximately every 20 seconds.
If the road is empty, you are obliged to drive right.
No exceptions.
It depends on culture, but e.g. in Germany it is not your business to dictate other people how to drive.
The most important law here is
the first law: Wer am Verkehr teilnimmt hat sich so zu verhalten, dass kein Anderer geschädigt, gefährdet oder mehr, als nach den Umständen unvermeidbar, behindert oder belästigt wird.
(People who are road users have to act in a way that no other people are harmed or endangered.
They also should not encumber or pester other people if this is not strictly necessary.)
And yes, driving left without reason other than being comfortable is considered encumbering.
travel.stackexchange.com/users/7442/Thorsten S.
If you are going "the speed-limit", in almost every location I've ever lived in the US, you are the slowest person on the road.
If you are the slowest person you aren't passing.
If you aren't passing and you are on the left, then you are breaking the law (where applicable).
If you are going above the speed limit, and other cars are stuck behind you then, sooner or later, you are going to make people pass on your right.
Whether you are "right" for actually following the speed limit or not is rendered moot by the fact that by not passing you are no longer following another law:
Slow traffic to the right.
I can drive 10 over the speed limit in most places and constantly get passed.
In many places in California, going 10 over on the highway would be the slowest lane.
Going 10 over in the fast lane would be asking for a road rage indecent.
travel.stackexchange.com/users/3389/WernerCD
Here in Ireland you will receive a fine and licence points, if you drive in the overtaking lanes but are not overtaking, regardless of your speed.
Overtaking lane is any lane, other than the left most lane.
edit
That is the law, implementation of this is almost non existent
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As the other answers have already pointed out, the laws vary greatly from one country to another.
Here in Spain you would be fined and I think that you will also lose points in your driver license.
I would like to point out that here overtaking over the speed limit is not illegal.
In fact, you can go faster than the speed limit when overtaking (by 20 km/h) in some specified situations, so, even if you drive at the speed limit, you could be always legally overtaken.
travel.stackexchange.com/users/33096/A. A.
As far as I have read, overtaking over speed limit is  always  illegal. So do I need to keep in left if I am at speed limit and no one can overtake me legally? It is also less tedious to just stick to one lane and cruise at a legal top speed rather than go to left lane, overtake, switch back to right lane N number of times.  This is of course keeping in mind that emergency vehicles are always given passage.
In countries with a "Keep right/left unless overtaking" rule: May I keep the left/right lane if driving at speed limit?
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<usa><europe><legal><safety><driving>
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travel.stackexchange.com/users/33485/Nandeep Mali
In mainland European countries like Germany and Spain, it is the law that you should occupy the free rightmost lane  unless when overtaking. Regardless of speed, you could receive a fine and licence points if you drive in the overtaking lanes but are not overtaking. You have to drive in the right lane if you are not overtaking. This is the rule almost everywhere.
You have to drive in the right lane if you are not overtaking. This is the rule almost everywhere.