Fares are regulated by law and taxi drivers are obliged to use a taximeter.
From the en.wikipedia.org
Thirty taxis were checked during the operation and three ordered off the road for tyre, brake and taximeter defects.
From the thisisbristol.co.uk
Anyone mystified by the way a taximeter clicks up your fare can now read about the formula that the meter uses.
From the newscientist.com
The first major innovation after the invention of the taximeter occurred in the late 1940s, when two-way radios first appeared in taxicabs.
From the en.wikipedia.org
More examples
A meter in a taxi that registers the fare (based on the length of the ride)
A taximeter is a mechanical or electronic device installed in taxicabs and auto rickshaws that calculates passenger fares based on a combination of distance travelled and waiting time. It is the shortened form of this word that gives the "taxi" its name.
An early 20^th century navigational instrument for taking bearings.