But those worse figures wouldn't be what VW advertised, they would be advertising the better 'regular' numbers.
And then you would demonstrate you don't know the law around those numbers.
Car makers have ZERO option except to publish the EPA approved numbers. They MUST publish the EPA numbers. The problem is the official EPA numbers are meaningless, derived from a fairly old process, and not indicative at all of actual mileage figures.
So, using those EPA numbers, hybrid owners have been really annoyed to find they're not getting anywhere NEAR the mpg they've been told -- because the hybrids were measured using the old and not-very-useful formula. Similarly, 15+ years ago, I knew people with diesel VWs. Those cars regularly got more mpg than they could advertise, because for those cars the formula was fairly useless in the real world as well.
The important thing here is that, right or wrong, high or low ... car makers can only legally give their mpg numbers based on an EPA formula which is, effectively, an estimate based on a calculation. If they tried to use other numbers they would get into trouble.
VW would advertise based on the only number they're allowed to. They can't cherry pick the ones they like; which means you could get significantly worse or better than the EPA figure. Even if the EPA figure is pretty much known to be meaningless and out of date.