First off,this goes back when I had three trucks
I have worked it both ways,the truck that gets the 60% and fsc actually makes more money than the one getting the 40%.At 40% there is a restriction on coming home.If over 150 miles,they would have to pay for the fuel.For that reason,there getting home is harder.
When drivers get the 60%,they do have to pay attention to what they are doing.More fuel they save,the more money they make.In a tractor fsc is based on 6mpg,when they get 7 mpg,thats like a bonus,5.5 mpg,its a loss of pay,so they would need to change what they are doing.
I've worked this out with the truck I'm driving now,and it still is the same.
Why do I know it works,thats how I pay myself.
drivers split 40%,truck get 20% plus fsc
Also,I have had drivers at 65% because truck thry were driving wouldn't get the minimum fuel mileage,tractor needing at least 6 mpg to get the minimum fsc
I at one time had a truck that wouldnt get the 6 mpg no matter how you drove it,so I kept that truck for myself,but I did a 65% and it worked out fine