The thing is that the cheats (throwing out unnecessary weight, taping door gaps etc, using super hard notrogen inflated tyres, etc.) would add a constant amount to the fuel consumption, say .2 liter per 10km.

If you keep pushing your engine consumption downwards to the ridiculous levels we see now (<.5 liters 10km), what happens is the "drag cheat" actually becomes a large percentage of the consumption.

Just goes to show that the cars should be designed without large door gaps or outside mirrors (cameras instead) and so on, since they seem to add significantly to the consumptions that that is where the effort should be put.

As a consumer, I really don't care much, I just get consumption data from some independent source such as a magazine or website. Having manufacturers self-test and self-regulate reminds me of the financial industry in 2008. It just won't work.