How is l/100km calculated?

[alkit]

Hi,
My car is currently reporting an average of 8.2l/100km. I have a 45l fuel tank. If my calculations are correct, I should have got around 549km from the tank.

Why then, after 320km is fuel tank on empty and fuel warning light is on? That's nearly 14km/l.

What am I missing? I doubt my car is using so much petrol?! And why then would average fuel consumption be repoting 8.2?

 

[the graduate]

do you have "stop-start" technology on your car...?

if not, then you might be going at 10-12km/l in heavy traffic...

 

[JK8]

You have a Polo?
The readings are always fluctuating.
Drive it hard till you using about 13l/100 then check your range.
I get about 450 when the light comes on.
My reason are mainly because it uses a lot in traffic and start up, over 10l/100km.

 

[kiepie]

Sorry for asking a stupid question. Did you reset it after filling it up again.

Is that "average" changing as you pull away and accelerate or does it only change every now and again?

My car has an average (the average over the whole trip) and a current (which goes from 0l/100km on stand still to 18l/100km on a pull away)

 

[the_don46]

The car has a 45 litre tank but probably close to a 7 litre reserve regardless of what the manual actually says.

Fill up your car to first "click"

Next fill up do the same thing. Stop. Take the mileage done between fill ups and divide by the litres of fuel just added. This gives you the kilometres per liter.

Take this number and divide it by 100. This gives you litres per 100km.

 

[iCubed.Saajid]

Hi,
My car is currently reporting an average of 8.2l/100km. I have a 45l fuel tank. If my calculations are correct, I should have got around 549km from the tank.

Why then, after 320km is fuel tank on empty and fuel warning light is on? That's nearly 14km/l.

What am I missing? I doubt my car is using so much petrol?! And why then would average fuel consumption be repoting 8.2?

Are you sure you're not confusing litres per 100km with kilometers per liter.

8.2 liters giving you 100km, is equivalent to
1 liter giving you 12.2km (100/8.2)

14km/l = 7.14km/l (100/14)

 

[Fazda]

It is OH so simple!

Take a calculator with you when next you fill up, and then....

Take the number of litres used

Divide that by the number of kilometers driven (That is if you were clever enough to zero your odo when you last filled up)

AND VOILA!!!!!!!!! you will get an answer that gives you X# litres/100 km

Maybe it's too late at night, but I cannot understand why people find this so difficult...we've been working out l/100 since the 70's for heaven's sake!

 

[UnUnOctium]

Hi,
My car is currently reporting an average of 8.2l/100km. I have a 45l fuel tank. If my calculations are correct, I should have got around 549km from the tank.

Why then, after 320km is fuel tank on empty and fuel warning light is on? That's nearly 14km/l.

What am I missing? I doubt my car is using so much petrol?! And why then would average fuel consumption be repoting 8.2?

Calculates it by the PWM signal sent to the injectors. If you know RPM and how much each injector is spraying, you know how much you're eating. 1) when did you reset the average? 2) I don't think consumption at idle gets factored into that one 3) You don't always use up your full tank. To really measure it, fill your tank up, keep receipt, fill up again after driving, see how much you filled up and calculate the distance and then simply calculate how far you drove from the last refil. That should be equivalent to the average (presuming you reset it on the first fill up). P.S. Make sure you fill it up to the brim both times.

 

[Fazda]

Calculates it by the PWM signal sent to the injectors. If you know RPM and how much each injector is spraying, you know how much you're eating. 1) when did you reset the average? 2) I don't think consumption at idle gets factored into that one 3) You don't always use up your full tank. To really measure it, fill your tank up, keep receipt, fill up again after driving, see how much you filled up and calculate the distance and then simply calculate how far you drove from the last refil. That should be equivalent to the average (presuming you reset it on the first fill up). P.S. Make sure you fill it up to the brim both times.

That even confused me, and I've been working out l/100 km since Noah fell off the Ark!

 

[Nerfherder]

It is OH so simple!

Take a calculator with you when next you fill up, and then....

Take the number of litres used

Divide that by the number of kilometers driven (That is if you were clever enough to zero your odo when you last filled up)

AND VOILA!!!!!!!!! you will get an answer that gives you X# litres/100 km

Maybe it's too late at night, but I cannot understand why people find this so difficult...we've been working out l/100 since the 70's for heaven's sake!

No that will give you l/1km

Its like this:

You put in 45l and do 450 km (example), then you divide both by 4.5 to get 10l and 100km.

This gives you 10l per 100km

If you want to out the one over the other then you must still x by 100 to get the answer.

 

[milomak]

so we know it is reporting an average. but over how long? is it a trip usage or average over the last 10km or whatever they use as their basis.

look in your manual and it should tell you the parameters it uses.

much like sometimes you may find an average speed calculator actually shows the average odometer speed as opposed to the true average speed.

 

[UnUnOctium]

That even confused me, and I've been working out l/100 km since Noah fell off the Ark!

That. Except I tend to do mine by filling it up to the maximum since different pumps have different cut-off points whereas your car's maximum capacity doesn't change

The car has a 45 litre tank but probably close to a 7 litre reserve regardless of what the manual actually says.

Fill up your car to first "click"

Next fill up do the same thing. Stop. Take the mileage done between fill ups and divide by the litres of fuel just added. This gives you the kilometres per liter.

Take this number and divide it by 100. This gives you litres per 100km.

 

[alkit]

Thanks guys for all the responses.

I apologise, but I don't think I phrased my question correctly, it should be like this:

My car claims a combined average of 7.7l/100km. According to my onboard computer, I have averaged (since last tank full) at 8.2l/100km. That seems 100% ok.

What I don't understand is why my tank is showing empty at 320km. I should be able to get over 500km on a 45l tank...

That means I am technically using around 14l/100km, so to me it seems like the engine is gulping petrol and the onboard computer is very very inacurate.

So my q is - why am I not getting over 500km off a tank like both the specs and computer of my car are reporting?

 

[Fazda]

OK, you are dealing with possibly the most inaccurate instrument in a car.

The fuel gauge reading is taken from a "sender" in the tank, which is similar to the ballcock in a toilet , so as the "arm" drops it will register a lower reading on your dashboard.

Some of these will stick at full for 250 k's and then collapse over the last 200 km, whilst others are relatively accurate. The only way to see what mileage your car is doing is to fill up, drive for a while, and then fill up again, and calculate your consumption from that. the onboard computer is not always very accurate in some cars. I have seen a variation of over 1 l/100 on the VW Polo, whereas my Subaru is dead accurate.

Rather than trying to squeeze every drop of gas out of a tank, it is actually safest to fill up when the petrol is between half and empty. That way you know that you aren't going to run out.

BTW, Citi Golfs with a blocked breather can and do suck their tanks in on themselves when driving from the coast to the highveld. I lost 2 tanks a few years ago...then the tank dropped from a 45 litre tank to about a 10 litre capacity...then you are guaranteed to run out!

 

[SlinkyMike]

It is OH so simple!

Take a calculator with you when next you fill up, and then....

Take the number of litres used

Divide that by the number of kilometers driven (That is if you were clever enough to zero your odo when you last filled up)

AND VOILA!!!!!!!!! you will get an answer that gives you X# litres/100 km

Maybe it's too late at night, but I cannot understand why people find this so difficult...we've been working out l/100 since the 70's for heaven's sake!

I don't think it is the calculation itself that is confusing but rather the definition of 'average'.

If you drive the same way to and from work everyday with no fluctuations in traffic and no deviations in terms of aggressiveness of driving and route it might be clearer but really... who does that?

I think a few km variation from what the onboard computer says is quite expected.

 

[bullzeye.za]

No that will give you l/1km

Its like this:

You put in 45l and do 450 km (example), then you divide both by 4.5 to get 10l and 100km.

This gives you 10l per 100km

If you want to out the one over the other then you must still x by 100 to get the answer.

No, no no..
You don't divide them both by the same number..

The formula says liters per 100 kilomiters.

So you already have liters ( 45 ) and you have kilometers ( 450 ), but you want to know how many 100 kilometers, so divide the number of kilometers by 100.

So you now have 45 liters per 4.5 hundred kilometers, 45l/4.5(100km) = 10l/100km

I hope that made sense.

 

[Fazda]

I don't think it is the calculation itself that is confusing but rather the definition of 'average'.

If you drive the same way to and from work everyday with no fluctuations in traffic and no deviations in terms of aggressiveness of driving and route it might be clearer but really... who does that?

I think a few km variation from what the onboard computer says is quite expected.

People are also very quick to read road tests where fuel figures are given and expect to come out on those figures. The chance of your car producing the same figures as a test car is highly unlikely. In some cases you will do better, and in others, much worse.

A road test figure simply gives you a ballpark idea but can never be stuck to in every day driving. If you take the daily trip into work, all you need is one accident or one traffic light on the blink and your figures can go to hell.

 

[Nerfherder]

No, no no..
You don't divide them both by the same number..

The formula says liters per 100 kilomiters.

So you already have liters ( 45 ) and you have kilometers ( 450 ), but you want to know how many 100 kilometers, so divide the number of kilometers by 100.

So you now have 45 liters per 4.5 hundred kilometers, 45l/4.5(100km) = 10l/100km

I hope that made sense.

LOL... you just did the exact same thing as me. The 5.4 is just to convert the 540km to 100 km.

The maths is right...

 

[Fazda]

Here is an actual example.

Km driven :432.6

Litres used : 39.75

Consumption = 39.75 /432.6 x 100 will give you 9.188 l/100

 

[SlinkyMike]

People are also very quick to read road tests where fuel figures are given and expect to come out on those figures. The chance of your car producing the same figures as a test car is highly unlikely. In some cases you will do better, and in others, much worse.

A road test figure simply gives you a ballpark idea but can never be stuck to in every day driving. If you take the daily trip into work, all you need is one accident or one traffic light on the blink and your figures can go to hell.

Exactly.

I think people actually take some of this stuff way too literally... like truck drivers in the UK driving into lakes because the GPS said so.