Well, grass consumes carbon dioxide and outputs oxygen. But mowing the grass consumes oxygen and outputs carbon dioxide. And of course mowing the grass consumes fuel and outputs various air pollution.
So what's the most efficient way to mow the grass ?
First consider the engines. In general, an internal combustion engine gains efficiency with increased valve area over the displacement. So a twin-cylinder 500cc engine is likely more efficient than a single- cylinder 500cc engine. More likely, the buyer will find something like a twin-cylinder 660cc instead of a single-cylinder 590cc engine. But also the twin-cylinder engine will tend to be quieter than the single- cylinder engine.
Next is mowing width. And twin-blade riding lawn mowers can be found as small as 38" although a 42" will go through a small gate. And so does an 18 HP mower with a 42" mowing width use less fuel than a 10 HP mower with a 28" mowing width ?
Finally, there is a new type of transmission for riding lawn mowers. Well, a lawn mower runs wide-open because it has to cut the grass with the blades. And so a lawn mower changes speed by changing gearing and not by changing engine speed. The new hydrostatic transmissions vary the gearing with a hydraulic system instead of with mechanical gears or instead of with CVT belting. The result is that the mover can smoothly and effortlessly change speeds and easily switch between forward and reverse. Then mowing time is reduced as maneuvering is much easier. The hydrostatic transmission can be controlled by a lever or by foot pedals. A zero-turn riding lawn mower has a hydrostatic system for each rear wheel while a hydrostatic lawn tractor only needs one system for both rear wheels.
And so there is some amazing current technology for riding lawn mowers .