I'd make it depending on years, like I do in my folders. For example, if the Russian car was manufactured in, say, 1907, before the USSR has appeared, i feature it with [R] sign, Soviet cars - with [SU], modern ones - with [RUS]. If the manufacturer was active during several periods, I specify them all: [R, SU, RUS].
Soviet cars may not be specified as Russian, as well as on the opposite, we can't feature the car built in, say, Soviet Ukraine as Ukrainian IMO, as there was no such country during that period - it was a Soviet republic, while post 1991 cars would already carry Ukrainian sign. Of course, sometimes several countries of orgin can be applied for a single car. Korvensuu, first car built on Finnish territory, is most often reported to be a Finnish car, but in 1912-1913, when it was constructed, Finland was not a country, so according to my system it was a car built in Russian Empire [R]. If it became a company and remained car production after 1918, I would feature it as [R, SF]. Such cases can be found for more countries - Austro-Hungary, Germany, Czechoslovakia, Yugoslavia, etc. etc.
I think it is the most logical way of presenting this information.