My company has allowed the drivers to choose to either be "optimized" by the company, i.e. they set the maximum speed the truck will run, or to "self optimize", where the driver works toward keeping the average maximum speed at or under 70.
Optimized drivers get an extra .03 cents a mile automatically.
Self optimized drivers get the .03 cents a mile provided they keep the average maximum speed under 70 each month.
So far I have always been able to achieve that goal and earn the premium pay.
We have the exact same system with the same extra pay, only we have to keep it under 60 mph 93% of the time. I did it for a few years but now I just keep the pedal to the floor and run 68. My pay hasn't decreased with the extra miles I can run and my fuel mileage still averages around 6.9 mpg.