Why are there different tachograph charts rate 100 125 140 and 180?

Answer 1

Different maximum speeds (in km/h) for different classes of bus and truck, depending on the speeds allowed in various countries..

Answer 2

To Vir John, Another case of two countries separated by a common language.
A TACHOGRAPH is a SPEED measuring device (with a paper recording disc - about the size of a CD) fitted to commercial and passenger vehicles according to EUROPEAN transport law - in place of a standard SPEEDOMETER . A TACHOMETER is what we call a REV COUNTER.
As Twisted Ace says, it is for the various classes of commercial and passenger vehicles in the European Union and the various speed limits in force in those countries.

Answer 3

Tachograph or tachometer I call it tells the engine speed. Most tachometers has numbers from 0-8 and then below it indicates 'x1000RPM'. That's the actual engine speed. But it applies to gasoline engines because they can reach higher engine speeds unlike a diesel engine. Diesel engines has a tachometer ranging from 0-7 but the redline(excessive engine speed) is at 5000 RPM. Most of the time their multiplier is x1000. But in bigger diesel engines like trucks/buses have a redline not exceeding 3000RPM. For the people to think that the engine speed is fast, you will see in their tachometers ranging from 0-400 but the multiplier is only x10. It tricks the person looking at the tachometer to think that the engine has a high speed.
But that doesn't mean that low RPM engines cannot reach high speeds. It also depends on the torque(twisting force) of the engine. All diesel engines have high-torque engines. They can also reach high speeds, yet slowly. but that also depends on the transmission if it's designed for high speed or high torque. It's a bit long if you want me to explain further just give me a IM..