Estimate fuel from driving habits.
Driving style has a larger effect on fuel economy than most mechanical modifications. Hard acceleration followed by braking throws away the kinetic energy you just paid for, and the US Department of Energy puts the penalty at 15 to 30% of highway economy for aggressive driving. Sustained speed above 70 mph costs a further 10 to 20%, because aerodynamic drag climbs with the square of velocity. This calculator applies each penalty in proportion to how much of your driving fits the description, then converts the resulting economy loss into gallons — which is the correct order, since a 20% mpg loss burns 25% more fuel, not 20%.
Driving Habits Fuel Cost
Economy loss = aggressive share x 0.25 + high-speed share x 0.15; actual mpg = baseline x (1 - loss); extra cost = (miles/actual - miles/baseline) x fuel price
Between 15 and 30% on the highway and up to 40% in town. It is the single largest controllable factor in fuel consumption, ahead of tyre pressure or servicing.
On flat roads, yes — it removes the small speed oscillations that cost fuel. On hilly terrain it can be worse, because it accelerates hard uphill where a driver would let speed bleed off.