Well it's not necessary to run your car on water because it already runs on air. See the fuel is misted, mixed with air, compressed in the combustion chamber, and hit with a spark. Now a lean fuel mixture runs hot while a rich fuel mixture runs cool. So your air-cooled lawn mower is running a rich fuel mixture.
So just get rid of your yard grass to save fuel. One web source says that Liriope grows only 1" tall and will make a yard. Then Blue Fescue is also mentioned. Then they go to Clover...but hey I've seen Clover go to 4" high ?
Now there's an old joke about needing a car gas pedal that pushes back. And so one car maker is making a gas pedal that pushes back. I suppose it looks at speed and engine RPM and figures out if you are going up-hill or down-hill...and then it allows less throttle on the down-hill.
But the driver can restrict the gas pedal on their own. Let the car gain speed on the down-hill but use less gas pedal on the down-hill. Then on the up-hill don't worry about losing speed but just try to make the hill crest before the speed drops too far under the speed limit. When leaving a stop light be the first to pull away (with ease) but then let the other cars catch up and pass by...while looking ahead to the next stop light.
Now put air in your tires. If your car has the engine towards the front put
36 pounds of air in the front tires while if your car has the engine towards the back then put 36 pounds of air in the back tires. Next with a full fuel tank put air in the remaining tires to match the radial tire bulge of the tires already aired. Fuel savings will be 7.5%...Not much left that can be done...but a very large front spoiler or air dam will save fuel. Consider a wave breaking on the beach and realize that an air wave is breaking under your front bumper. Anything that reduces air flow under the front bumper will reduce drag. And it's often said that a fully skirted car has downforce but actually a production-based skirted vehicle just reduces lift..but again drag is also reduced. And slide through the air...
And then the rear spoiler helps a little as it pitches rearward the airflow that is coming over the car...
Oh save electric powerplant fuel and natural gas fuel. Just plant hardwood trees around the house. They shade the house in summer but let the winter sun pass through. Don't buy solar panels for the house unless you live in the desert...because the trees will lighten the load on your heating-and-air-conditioning system by an amount greater than the power produced by a homeowner's quantity of solar panels. Now willow oaks are popular because the leaves stay on the gournd after they fall and the acorns are small. And a weeping cherry is not a thick foliage tree but the branches are save to hang over the house. Of course Silver or Red Maples are good choices but don't let the branches reach over the house. (Pull the seed out of the Maple pod, boil it, and it tastes just like asparagus.) Then the Bradford Pair is not so good because the tree gets so thick that the limbs break off...even though the limb diameters are safe.
Back to cars. Buy a car with a four-valve per-cyclinder head. And that often includes mechanical variable valve timing. Then lust for direct fuel injection rather than port fuel injection. And look for 5 speed (and up) automatic transmissions. (Now a manual transmission gets better highway mileage than an automatic transmission but for the manual transmission to get better city mileage requires an expert or perfect driver.) Finally look up the weight of the car and consider the weight...