Ok, let me see if I understand this. So Nissan can claim that a 2017 Leaf can go 300miles on a single charge, then all they have to do is set a test procedure with the Leaf being towed behind a tow truck (external power source), and not disclose the test procedure to anyone. Then, when customers who get their new Leafs try to go court over false claimed range, they would have no legal recourse since they could also tow their Leafs 300 miles on a single charge?
In that specific example, Nissan's hands are tied because the EPA has a mandated test procedure for EV range (so regardless of what automakers advertise, the new car sticker is legally required to post a number based on that test procedure).
No such procedure exists for testing power (whether for ICE or EV cars). No US power standard exists for EVs. And in the ICE world, automakers can use SAE gross, SAE net, SAE certified (or even their own method) without explicitly specifying which they used. It is not legally mandated that horsepower tested in a certain way, be posted on the new car sticker.
However, assuming that there is no EPA mandated range testing, I'm not saying a consumer would have no legal recourse. I am saying the lawsuit is different between:
1) A claim that is literally false (as in the Ford Mustang Cobra, Mazda RX8, Mazda Miata cases)
2) A claim that is true but may be misleading (as in the Tesla case)
For a literally false claim, it is pretty straight forward: all the plaintiff has to demonstrate is a production engine doesn't make the advertised power under the test procedure used by the manufacturer to determine that number. Ford and Mazda knew they would lose because they tested using different parts and then changed it in the cars sold to the customers (while the test was inclusive of those parts).
In the latter example, you have to establish that the claim would be misleading to the target audience of the advertising, which is usually done using surveys. It will not be based purely on facts, but also on perception.