2022 Citroen C3 2022 c3 aircross eats6 auto

EAT6 I bought this Electronic Automatic 6 speed Transmission car because it was supposed to be efficient. Up to 5 gear it is OK (45mph) , but will not engage 6th gear unless exceeding the nation speed limit of 60mph close on 3000 revs. Citroen workshop telling me this is standard, and its my fault I don't use duel carriageways. (there is none within 70miles). Where do Citroen get their fuel consumption figures from? I am achieving 37.5mpg. Does anyone know what 6th gear lockup speed/revs should be.