Fuel is not wasted only at the pump. It often disappears in the gap between what an automatic gearbox can do and what drivers ask it to do with a lazy flick to D and a heavy right foot.
The harsh truth is simple. Many automatics are calibrated to keep revs low under light throttle, so when a driver wants to maintain speed and stays in D, the transmission often upshifts early, drops engine speed, and forces the engine into a higher load zone where throttle openings are larger and brake use is frequent. That behavior defeats deceleration fuel cut-off, the engine control strategy that shuts injectors completely when the throttle is closed and engine speed is sustained by the wheels during overrun.
Smarter coasting flips that script. By easing off the pedal early and letting the car roll in gear, drivers allow the torque converter lock-up clutch and engine braking to keep the crankshaft spinning without extra fuel, because injector pulse width can fall to zero during overrun. Short, anticipatory lifts of the throttle use kinetic energy instead of chemical energy, often trimming consumption by double digits without any hardware change.
Manual mode then becomes more than a gimmick. Holding a slightly lower gear during gentle deceleration keeps engine speed inside the window where deceleration fuel cut-off is active, avoiding needless downshifts from the transmission when the driver panics and stabs the brakes. Light, steady throttle inputs, combined with deliberate gear selection, let the transmission logic act as a fuel gatekeeper rather than a passive accessory.
