Realistic Thoughts on Improving Your Car's MPG


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. There has been a lot of talk about acetone and mixing it with your gas when you fill up. The general consensus is that adding 3oz of acetone per 10 gallons of gas is the ideal mixture. I have used acetone with different results and I have pretty much came to the conclusion that a properly tuned and maintained car will see no improvement with adding acetone to your tank. But the only real way to find out is to run it for a few tanks and see what happens. Typically it makes a good fuel system cleaner so it is worth using occasionally anyway. Once you start using it the first tank will normally be a bit worse mileage if your fuel system isn't clean. On the third tank is where I would decide whether I would continue using it. After the third tank you could run 2 tanks without it and see if your mileage drops back down. It could be that the acetone cleaned your fuel system and now that it is cleaned the acetone is no longer needed. If the car is not tuned properly from the factory the acetone can improve mileage under the right circumstances so if it works for you then by all means continue using it. It will not damage the car in any way when used in the 3oz per 10 gallon range.

Next up is water injection, I am not going to explain how to build it, there is a detailed article on the mother earth news website for how to build it. Water injection works and if tuned properly to use 1 gallon of water for every 10 gallons of gas used it will help your mileage by about 3-5%. I recommend using windshield washer fluid instead of water. It gives a slightly better gain. The hard part is keeping the fish tank regulator adjusted so it mixes properly. Also make sure it is using a ported vacuum source or it will idle really badly. I gave up running it because it wasn't worth the hassle on my car trying to remember to keep the small jug I was using full and keeping it properly tuned.

There are other modifications you can make, advancing your cam timing can help improve mileage but it will make the engine have less power. If you have a distributor you can advance the base timing slightly and improve the mileage. If you ever have the oil pan off adding a crank scraper and windage tray helps at the higher RPMs. You can shave the head slightly and add Singh groves to the combustion chamber. Changing your differential gearing to a taller gear helps mileage as well as going to a taller and narrower tire. Both of those can throw your speedometer off so don't do them unless you are prepared to deal with that.

There are a lot of things you can do to improve your mileage, it is just a question of how much you are willing to give up in your car. You can't get better mileage for nothing, so after reading this article it should give you some ideas on what you can do. There are a few websites that are good to help give you ideas on getting better mileage. is probably the best one around so look around there if you want to learn more. You can also look at what I have done to my Metro XFI for some additional ideas.