My point is:

Fuel savings are a benefit to the owner/operator and are difficult to measure. At best 20%.

CO emission reduction is of benefit to the environment and is easy to measure (90%), just set a sensor over the weigh-bridge and have the emission checked every time the truck passes through. Basically, it's either got black smoke or it doesn't! If the truck has been on a dyna test at the shop then simple ongoing tests should be all that are required to ensure that the truck is still compliant.