I have done a lot of experiments and have found some things that consistently work and some things that actually hurt your mileage. I am going to describe here what I have found that works and what doesn’t work. This is all based on experiments I have performed on my Geo Metro XFi.
The first thing you need to realize when looking into improving your mileage is that you can’t realistically double your car’s mileage no matter how hard you try without modifying your driving style. At best you can get maybe a 25% improvement in mileage. If there was something that was easy to do that would improve your car’s mileage with no real expense then don’t you think it would be included already from the factory? Most of the stuff here will change the looks of the car or change how it drives. There is no real secret to any of it, when you change something you are giving up something in return for more mileage. What you are giving up could be comfort, quietness, or the looks of your car. Anyone claiming that you can buy their product and get X% better mileage with no other real change to your car other than bolting it on is just trying to take your money.
In order to see what actually helps your mileage you need to calculate it on every tank. Keep a small notebook in the car and write down the miles and gallons used, also write the approximate temperature range that the weather was for when you were driving that tank. The weather changes your mileage, the colder the worse your mileage so it is handy to know when the temperature changes in your log. It is a good idea to go over the car and make sure everything is working as it should. Check your tire pressure and set it to the max sidewall pressure and ignore what the car’s recommended pressure is. It will ride harsher but get better mileage. Make sure your plugs and wires are in good shape and that your fuel filter is in good shape. the air filter isn’t as important on a fuel injected car but if it is really clogged up it can hurt your mileage and power.
The easiest thing you can do to improve your mileage is to slow down, accelerate easier and try to minimize using your brakes. The faster you drive the worse your mileage is, it is as simple as that really. Being smooth and timing your stop lights and start coasting farther back when you are going to stop. If the car is an automatic then find out what speed the torque converter locks up and and try and keep it over that speed. If it is a manual then drive in as high a gear as possible without lugging the engine.
Next up is get rid of excess weight. Dump all the crap out of the trunk, all the unneeded stuff can go if you want to take it to an extreme. Remove the back seat, passenger seat, seatbelt hardware for those seats, Scrape up the sound deadening material from the floor, and take off the carpet backing material. The spare tire can be taken out and replaced with a pump and plug kit, those will fix most of the flats you have typically. You could get carried away and strip out lots of other stuff like the air conditioning, power windows/locks, extra interior panels, and swap the side and rear windows for Lexan windows. Getting out the excess weight will also improve your power and make the car accelerate much better. If you drive easy and don’t use the extra power you gained you can pick up a few mpg from lightening your car as much as possible. A rule of thumb is for every 125lbs you drop you can pick up 1mpg.
If you drive a lot over 45mph then doing some basic aerodynamic changes can help. Most cars have a lot more air coming in through the grille than they actually need. You can block it off and leave a smaller opening and the car will run fine but improve your aerodynamics. Adding a lower front air dam is also a very good improvement. I have used lawn edging available from any hardware store and trimmed it and fit it onto several cars and it actually looks good if done carefully. If the laws in your area are ok with it, taking the passenger mirror off can help your mileage. It is one less thing sticking out in the wind that way. Also if you have a luggage rack remove it and seal the holes with silicone sealer or something. Any spoilers or wings on the car will help if you remove them. If you want to go nuts then adding a rear wheel skirt and smooth underbody cover will also help. Those really change the look of the car though so they aren’t for everyone.
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