A good fiend who drives tractor trailers for a li9ving explained the "Elephants" driving side by side, with a 1 MPH difference.
Computerized engine controls limit the top speed, as selected by their company.
The cruise controls are a separate function, and can be 'bumped up' to a speed higher than the foot feed limit. If you touch the brakes, and drop in behind another truck, it will take many miles to get the cruise control back to that higher speed.
Staying on cruise can be an extra 2 to 5 % of pay for pay by mile drivers.