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.

The thing that makes this a pretty easy mod that can have a measurable impact on your mileage is that the sensor lets you define what lambda should be using a laptop computer to program it. So lets say you install the sensor and use a laptop to look at your AFR (air fuel ratio) and it shows the computer is easily able to maintain 14.5:1 which will give you normal economy. The computer is listening to the Oxygen sensor and adjusting the AFR to keep it as close to that ratio as possible. The LC-1 can trick the computer by saying the car is above or below that number. So you can program the LC-1 to say that 15.0:1 actual AFR is reported to the computer as 14.5:1. This makes the car run a bit leaner and uses less fuel. How much you can push this beyond that number will take some trial and error but it is pretty simple to change the value the LC-1 reports.

I would recommend installing the LC-1 and leaving it at 14.5:1 and take it for a drive to make sure the car’s computer has no problems with it and does not show any error codes. Once you are satisfied the car is running properly with the new sensor then adjust the ratio slightly leaner and see how the car responds. Maybe go to 14.8:1 and drive it. Once you are satisfied that the car will behave and work like it should then you can start testing the limits of how lean you can run your engine. It is possible if you drive easy to run the car as lean as 18:1 with no adverse effects. The biggest problem you will have is the car will start to knock if it gets leaner than it really can handle. Listen for this to happen and if you hear it then adjust the LC-1 to be a bit richer. I would recommend filling up the tank and driving a full tank at something like 15:1 and checking your mileage. Then go in small steps to leaner AFR each tank until your mileage starts to level off. Once it levels off go back down to your last setting and run it there.

Just remember that if you have to take the car in for an emissions test that you may have to adjust your numbers to make the car pass. This really depends on the car so there is no way to know if it will pass or not before hand. You could play it safe and set it back to 14.5:1 before the test but then you would not know if it can pass running leaner.

Do not get greedy and keep leaning the car out beyond where it runs well. Even though the increase in mileage will diminish after you go past a certain point leaning the car out it will still get slightly better mileage so you might be tempted to lean it out even more. This can damage your exhaust valves by overheating them badly and will be expensive to repair so don’t do this! Also don’t blast your radio while you are testing this out or else you will not be able to hear what is happening with your engine. This can improve the mileage of pretty much any fuel injected car but you have to use some common sense and don’t try pushing the limits of your car since an extra gallon of gas is way cheaper than a new motor.

The curve is not perfect; I am pretty sure the 5 degree advance number is a bit lower than what it should have been. I would say advancing a stock XFI engine 5 degrees is ideal, and advancing one with the head shaved 3-4 degrees is probably ideal. I will redo this experiment later on going in one degree increments from 2-5 degrees advanced to see if I can fine tune my timing.