There is no fuel mileage requirement from the government, in this range. If there were, they would cancel all the full-sized trucks and SUVs. There is a corporate AVERAGE fuel economy requirement, but no requirement for specific vehicles. Fuel mileage would/could have been addressed in the redesign that was scheduled. I'm sure it was before you, but when we were back over at Trailvoy, there was an Engineer who worked for GM truck at the time that this all went down (billdaman was his screen name). He had posted on one of the "why are they cancelling the redesigh?" threads that marketing had already OK'd the redesign, and wanted the redesign, but the head of GM global engineering, Jim Queen (known as alfalfa) convinced the brass above marketing's heads that mid-sized uni-body SUVs were where it's at for the US market, and the 360/370 line was stealing business from the more profitable full-size SUV line, and that the redesign should be cancelled. The brass agreed and over-ruled marketing, and the TB died. Over-seas engineering actually did the redesign several years later, which is what we're seeing now. I remember him saying something about that the Lambda was supposed to be another option along side the TB, not replacing it. The Lambda platform was actually designed to tow 3k#, but when Lutz came down with Queen and cancelled the 360 platform, word was that there was a gap in the lineup, and the Lambda had to be rated to tow 4k#. A few days later, nothing had changed in the design, but the Lambda platform was now rated to tow 4k#. It was much more political inside GM than technical, or governmental.
Mike