In France the CV number, is used to determine the costs of registration and the yearly car tax (aka "vignette"). CV is originally the abreviation of "Chevaux Vapeur" which can be translated as "steam powered horse".
The registration costs are proportional to the CV rating (about 180FF/CV or US$30/CV), but the tax is highly nonlinear dependend on the CV-rating and also depends on where one lives. This was not the case before 1981 when the socialist were elected, they wanted to promote decentralisation, and they used the "vignette" to fund that.
For that reason, most French car owners favor cars with a CV-rating at or below 7 (this also helps resale since most second-hand car buyers are more cost conscious than new car buyers).
The local variations in the "vignette's" price are also the reason why most (if not all nowadays) rental cars get registered in Marne (and thus have plates that bear a 51 number): the local "President du Conseil General" is against car taxation (as a matter of principle); and since he cannot legally get rid of it completely he tries to have the lowest possible rates.
Thanks to Jean-Marc Calvez for the equations and explanations on this page.
During this time the fiscal power was proportional to the engine displacement of the engine of the car.
Since 1977 (as a consequence of 1974 oil crisis) the fiscal power calculation depends on the engine type (gasoline/petrol or diesel), the displacement, the gear ratios and even (indirectly) the tire diameter.