The definition of a microcar has varied considerably in different countries. Since there are usually tax and/or licensing advantages to the classification, multiple restrictions are often imposed, starting with engine size. The Register of Unusual Microcars in the UK says: "economy vehicles with either three or four wheels, powered by petrol engines of no more than 700cc or battery electric propulsion, and manufactured since 1945". The Bruce Weiner Microcar Museum (the world's largest collection of Microcars) says "Engine sizes of 700cc and less and 2 doors or less" and the US-based Vintage Microcar Club simply defines it as 1000cc or less.