OK< I searched on duty cycle now what?
Cite? Any time you have braking engine or wheel (it makes no difference), you are wasting fuel. Hybrids with re-gen braking don’t count. Anytime you brake energy that you extracted out of your gas is being used for something other than propelling the car. In the case of wheel brakes it is turned into heat and wear on your brakes. In the case of engine braking you are doing work to turn the engine against compression. To get the best gas mileage, you want to use as much of the energy from the gas for movement as possible, not wasting it by braking.
As far as speeding up past your target speed and coasting, besides the engine braking issue, you have to contend with wind resistance. Wind resistance is not linear.