No, this is wrong.
Edit: To clarify, it is 100% wrong to say that fraudulent misrep is a crime. It is not. It is a tort (civil wrong). Triable in the High Court or county court depending on the amount claimed. Never the magistrates court.
As to the rest of what you say, I strongly disagree. A salesman should not be stupid enough to effectively guarantee a particular mpg, but if he did then if the customer relied on it then his employer will be liable.
Just to point out I am a practicing litigation solicitor qualified for 15 years, and I do know what I am talking about.
I think we will have to agree to disagree. I have been involved in contracts for a long time and this would not constitute a guarantee as there are too many variables which are accepted under industry standards. A bit like wind farm and solar panel outputs, claims are made based on assumed output.
I should have been clearer on what I meant about fraudulant misrepresentation, outside of contract law this can form part of criminal law. Like the telecoms company that targetted a school and netted several million pound through fraudulant misrepresentation. There was also the timeshare people who were fined in a Magistrates court after admitting fraud through several counts of fraudulant misrepresentation.
But anyway we will just disagree.