How far above the speed limit is safe to drive?

They do 140 on the Autobahn. So, sky is the limit with a car built for it, ideal conditions, no curves, no other traffic.

  In a practical sense, I don't  speed in the cage. Dont need a Ticket.

 I do 5 mph over posted on my 'sickle.  Or, motor lugs.  80 on the freeway is actually safer (Our max limit is 75), as, the faster, the more inertia and gyroscopic precession. Accelerating into a curve produces centrifugal force,   pushing  tires down. Cops here don't chase bikers Unless Real  stupid. These are usual Ninja riding 20 yos.

that 140 is kilometers or 70 MPH

The answer depends on:

-the driver

-the vehicle

-the road and road conditions

-the weather conditions

-the amount of other vehicle traffic on road

-etc., etc., etc.

The basic speed limit is determined by the maximum speed limit and defined not by signs posted beside the road. It is defined by the road conditions, vehicles condition and drivers ability.

To say how much above is not a true assessment of risk. The safe speed might be below the posted limit.

Driving in the day light on a section of four lane high way in the desert is one thing. Driving a WV road in the middle of a ice storm is totally different. Driving in an area with a lot of kids is different from that stretch of road in the desert too.

In U.K., Anything more than +20mph is risking your licence. 100 mph + definite ban . 

It depends on the road,  weather conditions and whether it's an urban area.

Some speed limits are too high for the conditions, others too low. For instance, here in Britain 70mph limit is too slow so you'll find most cars travelling 10 to 20mph over the limit, while 30mph in congested towns is too high and 20mph is a safer option with kids and mothers crossing roads, not to mention pedestrians engrossed in their smartphone and not paying attention. 

Basically,  drivers should use their discretion and treat speed limits as advisory especially in town.

It depends on the car, the tires, the road conditions, the traffic conditions, the visibility and the competence of the driver. Sometimes it isn't even safe to drive the speed limit.

As long as you are staying with the flow of traffic, you are generally okay. Where I live, everyone drives 5-10 mph over the posted limit. They just do. Our streets are clear and dry and it never snows or freezes here. If you are going slower than that, you are a hazard, if you are driving faster, you are a hazard.

Depends on the vehicles.   How wide your tires are.  How hot and cold out side.  How accurate the suggested speed signs are. How wet or dry it is.    How windy it is.    How well you can smoothly turn.   What angle you take a turn.  ........

It's not "safe to drive" - as soon as you pull out of your driveway you are at risk, and are a danger to others.  Your risk drops somewhat on a freeway, but the likelihood of being killed if any accident occurs goes up quite a bit because you're driving faster.  The faster you go, the more likely an accident and the more likely it will be fatal.

Just tell yourself each time you drive that "today's the day" that you're going to have an accident,  Be on guard against it continuously while you're behind the wheel, and simply base your speed on that assumption.

If you're on a city's commuting network of freeways, just keep up with the traffic in your lane.  Don't be the fastest thing on the road, and don't be the slowest.  If you're passing a lot of cars, then slow TF down.  If a continuous stream of people are passing you, then speed up a bit.  Keep pace.

If you're on a lightly-traveled two-lane road or you're on an interstate between cities, just set your cruise control at the speed limit and leave it there.  You'll be safer and everyone around you will be happier.

Another stupid, immature and cowardly anonymous posting fool.

It is ILLEGAL to drive faster than the speed limit so it is NOT safe to drive faster.

Then of course if you had a brain and were a decent driver you would know that