You can't legislate accidents.
Wear and tear on cars
Fuel consumption.
Just because you think it's better for my car I have to drive my Honda Accord at 65.
It's my problem.
Also what research have you done on what percentage better fuel economy I would get at 65 v 75 in my Accord.
Or our minivan.
Pollution? Want to do something for pollution start by taking some of the old cars of the roads. Better yet make it illegal to drive any car that doesn't get... 20mpg (that's reasonable right)
Plant some trees plenty of space in Oregon.
The nightmare, which you have conveniently ignored. You would have packs of vehicles running together.
Which is already the case in Cali with the trucks at least now the cars aren't in the mix.
My suzuki got 28 mpg when averaging 65 and 24 when averaging 75. I tested this on level portions of I90 in Montana several years ago. Recently I drove my ford ranger to eastern washington, same route there and back. Got 28 mpg averages at 65 ad 26 at 75.
The pack driving is operator skill, not speed. Speed variance is a huge cause of accidents, if no one were able to drive too fast speed variance goes down. In Cali the trucks are limited to 55, if I were benevolent dictator the double nickel would be abolished, no split speed but no one can drive over 65. Hence all traffic on level roads is effectively limited to consistent speed