Fuel Economy FAQ

Hope this thread can help people with fuel economy problems. I'll sticky it for a bit and then add it to the combined sticky afterwards.

How do I workout my fuel economy / usage
Now a lot of people just go on the number of KMs they get per 'full tank' however this can vary depending on how much you fill up and how much you have left in the tank.
I use the following method to work out the fuel usage (Litres) per 100KMs (a more accurate and comparable figure due to variations in fuel tank sizes).

Steps for calculation fuel economy
1) When you fill up your tank, fill up all the way to the top.
2) Zero your trip meter and drive around as per normal
3) Once you've used up fuel (say half a tank or more) fill up again all the way to the top of the tank
4) Note how much fuel you put in this time and also how many KMs you've done.
5) Use this formula to workout your fuel consumption.

Fuel Economy (L/100KMs) = 100*(Fill up amount (L) / Distance travelled (KMs)

Example
So say you did 420KMs on 35L of fuel:
FE = 100*(35/420) = 8.01L/100Kms

What should my fuel economy be like?
Now this varies depending on your car, the condition of the engine and also the model of car you have. Look in your car manual for a rough figure (but note that these figures are usually quite optimistic).
In my 1993 Civic Si 1.6L DOHC non-VTEC car I get around 8-9L/100KMs depending on the way it is driven.

What could cause me to use more fuel than normal?
There are quite a few factors that affect fuel economy and this thread hopes to point out what things you can look at (starting from the cheapest things to inspect / change).

Tyre pressues
Having low tyres pressure creates more resistance and cause the engine to work harder. Check your tyre pressures and try running a slightly higher tyre pressure (check your owners manual or tyre packard inside the drivers door) for the correct tyre pressue.

Air filter
If you haven't checked/changed your air filter lately, take it out give it a clean and if its still OK put it back in. If however it needs replacing try replacing it.

ECU Reset
If your car has been recently serviced and is still using too much fuel, you could try reseting your ECU. Just pull out the ECU fuse for about 5 mins (or disconnect the negative terminal of your battery). When you start the car up, just let it idle for a few mins before driving off.

02 Sensor
The 02 sensor only really comes into play under closed loop conditions (ie free way driving or at idle). If your ECU is throwing a 02 sensor error code or the 02 sensor itself is faulty (you can check the voltage it should be sending to the ECU with a multimeter, depending on the AF ratio). If you know somoeone with the same car try borrowing their 02 sensor.

More to come or just reply here and i'll update..........

If the car's running really rich, you'll see a puff of fuel out the exhaust when you accellerate hard. Easily seen if you get someone to drive behind you. That can affect fuel efficiency.

There may be some sort of leak. But the fuel wont just disappear, you'll be able to see it on the ground or on the outside of the engine... has a distinctive reddish colour to it. Dont want it to drop down near the exhaust or it might get a bit toasty. Would have to be a serious leak to affect fuel efficiency though.

Just a couple of ideas.

Where are the ECU fuse and O2 sensor? do u have any picture of them? and how they look like?

What car have you got?

ECU fuse is in the engine bay near the battery. O2 sensor is on your exhaust manifold.

my car is EK sedan,?? do u have any idea? i feel piss of of my car now,, good drive only 350km MAX,, if i boost it a bit,, even less than this,, i mean is only a bit~~

Hey the resetting ECU thingo, BIG'O thread said to pull out and idle the car for 10 mins but yours says to just pull out for 5 min, then I assume put back in and then idle for a bit then drive off? Is that right?

You cant' idle your car if the ECU fuse is out :)

Take out.......wait about 10 mins.......plug back in and let idle.

what ive found is to slightly give the drive belts more slack (loosen the tensioners) and you can extra mileage on fuel. however the compressor, power steering, alternator will be affected.