Why are 40,000 kilometres the Earth's circumference?

You are right circumference of the earth is correct and that is what is used nowadays.

Earth's Circumference at the Equator: 24,901.55 miles (40,075.16 km)

Earth's Circumference Between the North and South Poles: 24,859.82 miles (40,008 km)

Earth's Diameter at the Equator: 7,926.28 miles (12,756.1 km)

Earth's Diameter at the Poles: 7,899.80 miles (12,713.5 km)

Average Distance from the Earth to the Sun: 93,020,000 miles (149,669,180 km)

Average Distance from the Earth to the Moon: 238,857 miles (384,403.1 km)

It wasn't King Louis. The political sponsor of weights and measures reform in the French Revolutionary National Assembly was the Bishop of Autun, better known as Talleyrand.

http://lamar.colostate.edu/~hillger/origin.html

By the same reasoning you could say that the Earth's circumference is 1 unit (or any other completely arbitrary number) and base your measurement system on that.

It makes more sense to base your units of measurement on something smaller (for example, the metre) that you can use every day with greater ease.

The measurement had to be as near to the yard as possible.

Because 40000 sounds a lot, so it makes the Earth sound big indeed.

just jump in your car,zeor your clocks and drive around the world till you end up back here then you'll have the answer ,you'll also no how long it takes too!!! which is something king louis didnt no.