I have a sat nav and use it in my Bongo and in my other car which is a Ford Escort,and they both show a difference of about 5mph.Sat Nav 50mph,speedo 55mph.Our friends have a Bongo and also a VW.They have the same sat nav as me and their readings are also 5mph difference.So I don't think it is only Bongos that are out.Probably got something to do with the satelites I suppose.
Ed