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2) None. Manufacturers spend millions of dollars chasing tenths of a percent, and there is a lot of incentive for them to improve things. The major losses in cars that exist now are (best I understand it) limitations put on engineers due to styling needs, power needs, or cost saving measures.
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I just spun my wheels for the past hour trying to look up fuel economy ratings for similar vehicles offered in the US, NZ, AU, and EU. Sadly, for some vehicles, I can easily find fuel economy ratings for the US, but not other areas, or the other way around.
I had a similar question arise in a US Federal court case (where I became officially recognized as a Fuel Economy Expert) back in 2008. Back then, I was more easily able to look at the VW Jetta Diesel, Subaru Outback, and other global models to compare regional fuel economy ratings. What I found was the US versions ALWAYS got
the worst fuel economy. European models faired 10-20% better than the US. AU/NZ models were rated at 20% to a
whopping 70% better fuel economy. (Specifically, Volkswagen had their Blue Diamond Diesel engine that was getting the equivalent of 72 US MPG Down Under versus 42 MPG in the US in the Jetta!)
This begs the question; do NZ/AU versions get worse emissions, or do US versions leave a lot on the table??
Though I cannot verify this, a friend of mine spoke with a Toyota engineer that claimed they can get well over 200 MPG US in their Camry (in the lab), AND MEET EPA EMISSIONS STANDARDS OF MAXIMUM EMITTED EMISSIONS TOXINS! However, the EPA tells them what fuel delivery system they must use, that they have to use specified catalytic converters, what software limitations they must stay within.... All evidence suggests the EPA has mandated a maximum fuel economy capability that must be enforced by the ECU -- at least in the US.