That would significantly increase the cost. The US isn't Europe, long haul trucking is traversing a thousand kilometers at a time on mostly empty flat highways.
That speed limit is widely flouted on rural highways in California and other States don't observe it at all. Typical max speed limit for trucks in the US is 70 or 75 mph, and a few States have 80 mph.
Wyoming's motto should be Drive whatever speed you like!
(It's not an exaggeration because there seem to be about 2 state patrol cars and they're always parked at Walmart getting coffee. If you want to drive 100 as private passenger vehicle, go right ahead. If you want to drive 85 with a rig full of cows, feel free. While Nevada and Montana lost their no speed limit statuses due to federal funding and regulations, but worked around it by issuing smaller penalties.)
Increasing the transportation time by 30% has costs throughout the supply chain that greatly exceed the small difference in fuel costs. They aren't even on the same scale. That latency directly adds to the production cost of everything the trucks are carrying.