How does a 5th wheel contribute to the accuracy of car speedometers?

[EEristavi]

I'm wondering how does speedometer work.
To be specific let's talk about car speedometer.
I googled it and found out that there are different kind of speedometers, but not quite understood it's working principle. My question is not about how it shows us speed (Mechanically with arrow or electronically with digits - well if it does matters...), but how it calculates.

 

[CWatters]

The process starts with the gearbox. There is a fixed relationship between the output rpm of the gearbox and the speed of the car. The relationship depends on any gearing in the transmission and things like the diameter of the wheels. The point is the output rpm of the gearbox can be used as a proxy for the speed of the car because of this "fixed" relationship.

The old mechanical type work as follows: An output from the gear box spins a magnet inside the speedo next to a metal cup/disc. This causes eddy currents to flow in the metal cup and the cup tries to spin around with the magnet. However the cup is restrained by a small spring that stretches and eventually stops the cup rotating. The faster the magnet spins the further the cup is dragged around and the further the spring is stretched. A pointer is connected to the cup and indicates the speed on a dial. The "calculation" is actually done when the car and speedo is designed. I think changing the diameter of the wheels usually causes the speedo to read incorrectly (but perhaps the spring in the speedo can be adjusted on some?).

The modern digital type (probably) use either an optical or hall effect (magnetic) device to generate an electrical pulse each time the shaft from/in the gearbox rotates. These pulses are counted by a microprocessor that calculates the speed. The program "knows" the relationship between the number of pulses per min and the vehicle speed. If you change the diameter of the wheels the car dealer can usually reprogram the speedometer to correct the error that would otherwise be caused.

 

[EEristavi]

So, here is the thing what I can't understand:
I have rpm, size of the wheel and I can calculate speed (it's school physics); but what if my car's wheels slips cause of mud, ice or my car is in the air for some reason. It seems more like we have some fancy tachometer than speedometer.

Maybe I understood incorrectly what you said...

 

[CWatters]

If the wheels on your car slip/skid then the speedo reads incorrectly. Fortunately that doesn't happen very frequently and when it does you don't normally care exactly how fast you are going.

 

[EEristavi]

Well thank you than.
I thought there was no error in such situations. So I've gone thinking how it was fixed. I had different ideas (which were not very easy to be applied).

And again, thanks ))

 

[CWatters]

When testing cars they sometimes fit a 5th wheel to the back. This wheel usually looks a bit like a bicycle wheel. It gives an accurate speed reading because it's not used to accelerate or brake the car so is less likely to slip.

http://www.pegasem.com/english/products_uk/fwl_uk.htm