I think the problem with the emissions test is it tries to be precise. The authorities seem to think that if we lock down all the variables, we get a good guess as to the actual figures. Which wouldn't be a bad way to do things if you were doing a science experiment.
If we're trying to avoid gaming the system, wouldn't it be better to drive around a bunch of cars, using the natural variability of the conditions to defeat the cheaters? Sure, it's not as precise, but you get better accuracy because people can't trick the system.
Have a large number of tests, let the law of large numbers rule (eg let 1000 random people borrow each type of car). Have a think about whether there's some bias between the manufacturers built in and adjust your sampling accordingly. Make the data available to scrutiny.
I agree. Paying people to do comprehensive tests in a few locations will probably cost less than a single brand new car, of which they're making thousands to millions. Even a thousand test drives is not that much; 20 cars borrowed for a month is more than enough to do those.
The essence of tightening emissions and CAFE standards (as currently implemented) is that they are hard to meet and even passing manufacturers are generally "just barely" passing.
For CAFE, makers sell some high mpg models at cost/slight loss to allow them to sell high-margin luxury and sports models.
If we're trying to avoid gaming the system, wouldn't it be better to drive around a bunch of cars, using the natural variability of the conditions to defeat the cheaters? Sure, it's not as precise, but you get better accuracy because people can't trick the system.