I can't believe a simple internet search cannot find what the class 8 fleet average fuel economy is but I think I found something buried under a different North American Council For Freight Efficiency report. They show the fleet average being pretty consistent from 2012 to 2018 the data they report. In 2012 it was 6.0 mpg, then it actually went down for the next few years to 5.8 (probably increased emission requirements) then it creeped back up to 6.0 mpg. I don't see any info newer than 2018 but certainly there is no 33% improvement in the last 10 years.
https://nacfe.org/annual-fleet-fuel-studies/#