Mfgrs always have R&D costs as a factor, so changing a regulation doesn't necessarily increase costs, just puts those R&D costs into different areas that need it.
It's a fact that R&D will always develop toward increased gas mileage because that will help sell trucks ... so, just because a regulation states a Mfgr has to do exactly what its already doing doesn't make the costs rise.
Not like the feds make a law that changes those regulations overnight - its usually a ten-year window for those kinds of change (cleaner air, for example) - regulations don't exactly sneak up on truck designers.
