I drive a Tractor Trailer for a living, and I must say on diesels anyway the EGR valves are the worst thing that ever happened to trucking from a horsepower and MPG perspective.
2003 and older models did not have an EGR valve installed.
The difference since 2004 has been more mechanical issues with system clogging up, less power and less MPG.
Although new trucks have worked a lot of the bugs out, there is still major problems with late model trucks.
A lot of owner operators are going back to mechanical engines with no EGR and finding their MPG and power increasing with fewer issues.
Cars and trucks with common sense technologies would have greater efficiency if not for emission standards.
We need to control pollution granted.
I had a 1986 Audi 4000s that got 48 MPG. When we have to buy hybrid system vehicles to get that same mileage we got in the 70 and 80's in cars, I don't believe the market in their mileage advertising.
I wish I had engineering knowledge. I would redesign the fuel system on my cars, and do away with emissions altogether.
If you think emission standards don't affect mileage, then look at European vehicles. They get better mileage and better performance, because their emissions standards are no where near what we deal with.
We could have much better cafe standards if the oil, car companies, and government were not in bed with each other.