I would have to disagree, based on the assumption that the customer said that was his average MPG. It is accepted practice that a sales person will provide an answer based on the information provided to him by the customer. In this case it is also accepted practice that a sale person will use official MPG figures to represent expected fuel consumption and in fact they are the only MPG figures he may quote by law. Assuming that the TA does 45mpg combined, his response would be deemed acceptable based on the information provided. The misrepresentation act would not expect a sales person to verify the information given by the customer and contradict it. It is not within the scope of the role of a sale person to do this. It would be the same as a customer going to a garage and asking for a 10' work top and then saying his tape measure was slightly short, the accepted standard for measurement is 10'.
In order to misrepresent something and have a claim on the person, they either have to fraudulantly represent it as something it is not or be negligent. The other form or misrepresentation is wholly innocent and the lapse of time begins from entering the contract.
In this case the sales person based his answers on the information given by the customer and also the accepted practice for quoting MPG. At best he could be accused of wholly innocent misrepresentation and I suspect a judge would say that there is no case to answer as the sales person followed accepted practice when responding to the customer.
I suspect you would have more chance of claiming from Fiat as the figures vary so much from the quoted figure, however again they are working to industry standards. So in terms of the law the figures quoted represent the economy using the industry standard tests.
By the way a fraudulant misreprentation could be heard in a Magistrates court as it is a criminal offence