The other problem of comparing older cars is that the quality of the petrol has changed. In the good old days there was no ethanol mixed in.
I have found that when I use a big named firm such as shell that the petrol burns a lot better and my consumption goes down. I asked a vehicle inspector ( Not TUV rather a firm that checks the petrol consumption for the official figures) about this and he told me that the petrol in Germany which you buy from the cheaper firms most often has a lot higher sulfur content. They have to use a very good petrol when doing these tests and not what is available on a lot of forecourts.
They are seeing it in the damage that is occurring to quite a lot of relative new cars with the head gaskets blowing due to the build up in the heads.