How to increase your lawn mower's fuel efficiency

The air filter is what keeps dust and other particles out of the fuel/air mixture that your mower requires to perform at its best.  Every air filter is different; some can be cleaned while others should just be replaced.  Your mower's manual will have details about your air filter, but in general it's a good idea to replace the air filter once a year.

On many mowers, the air filter is located on the top or the side of the mower deck (near the carburetor).  There will be a plastic or metal cover over the air filter, which is typically secured with one screw.  Once you take this cover off, you can easily remove the air filter to either clean or replace it.

As you mow, grass clippings and other debris can get stuck to the underside of the mower, where the blade is.  Grass can be especially sticky when it's wet.  When a significant amount of grass builds up, either on the underside of the deck or on the blade itself, it can impede the movement of the blade, causing the mower to work harder to mow at the same speed.  This increase in workload will result in more gas being consumed.

A good rule of thumb is to clean under the mower either before or after every time you mow.  With a pair of heavy gloves on, you can easily wipe the grass and dirt off.  If the mower is cooled off and you're not going to be using it for a while, you can even hose the underside down, but you may still need to pick a few pieces of stubborn grass off here and there.

Mowing more often uses more gas, but if you let the grass grow for too long in between cuts, it tends to get taller (and, depending on the type of grass, thicker) which requires the mower to use more force to cut it down.  If the grass is too tall or thick, you may be forced to mow slower or to stop occasionally to prevent the engine from stalling out, which will also use more gas than normally required.  Mowing once every week or two (depending on the grass) is typically more than sufficient to keep the grass from getting out of control.

Mowing wet grass can be a pain -- the grass doesn't stand up straight and you won't always get an even cut -- but it can also use more gas, as the engine works harder to cut through the grass.  To save on gas usage, wait until your lawn is dry before mowing.  The engine will have a much easier time, and the yard will look nicer, too.

On most mowers, the oil (which lubricates the engine's moving parts) should be changed once a year or so.  Over time, the oil builds up higher levels of dust and dirt, which can impede the movement of various parts as well as cause friction and wear.  You can either change the oil yourself (if you're comfortable doing it) or have a repair shop or lawn mower mechanic do it for you, but it is a good preventive measure that will improve your mower's fuel efficiency and life.