This is How to Calculate Fuel Efficiency

Dhaham · May 22, 2021

Dear Friends

This is not a question, but an answer to a question which was not asked. There is another big thread on this forum on "Fuel Efficiency" where large number of members had posted comments on fuel efficiency of their cars. But looking at some comments, we can clearly identify that, some of the members do not know how to calculate the fuel efficiency correctly. That is why I thought of posting this.

1) Get your car full tanked. Take down the current mileage; say M1.

2) Drive the car until fuel level becomes somewhat low. At least below the quarter of the full level.

3) Re-fuel again to full tank and take down mileage M2 and amount of fuel pumped V2.

4) Now, it is clear that, to move the car from M1 to M2, it has consumed V2 amount of fuel.

Therefore, the fuel efficiency = (M2 - M1) / V2

There may be some fellows who are not familiar with this algebraic formula. For the ease of them, I will try to explain using a numerical example.

1) Get your car full tanked. Take down the current mileage. Suppose it is 76,369km

2) Drive the car until fuel level becomes somewhat low. At least below the quarter of the full level.

3) Re-fuel again to full tank and take down mileage and amount of fuel pumped.

In this time, suppose the mileage is 76,801km and amount of fuel pumped is 45L.

4) Then the distance the vehicle traveled using 45L is 76,801km - 76,369km = 432km.

Therefore, the fuel efficiency is 432km ÷ 45L = 9.6km/L.

This method gives you the average fuel efficiency between these two fillings. But if you need a further accurate figure, you can extend it to multiple number of fillings rather than two. For example if you had five fillings then the formula will be (M5-M1) / (V5+V4+V3+V2)

Please note that the amount of fuel pumped at first filling (V1) is NOT used in the calculation.

iRage · May 23, 2021

1. You need to clarify that full tank means to fill up to the first auto release (the tuck you hear as the pump handle releases and stops). Otherwise you will not be able to make sure how much of fuel was actually put in (to match up to what was there before and used).

An easier way would be to just run it for an easier mileage like 100km or 200km or something (usually 100 is the easier one).

Dhaham · May 23, 2021

On 5/23/2021 at 12:54 AM, iRage said:

You need to clarify that full tank means to fill up to the first auto release (the tuck you hear as the pump handle releases and stops)

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Thanks for your comment.

I think, another method is, we can fill up to the lid.

Anyway, it has to be ensured that the car is on a flat, horizontal surface.

Sometimes I have heard some people are saying, "that Vitz is doing 15-16 in Colombo and 20 in out stations".

Also heard something like "My Alto / Mira is doing 18 in city and 24 in long trips".

Most of this kind of words are due to not knowing how to calculate the fuel deficiency correctly. Otherwise, they are telling lies.

mazdaspeed · May 23, 2021

Nice post Dhaham

Yup that's how I general do it - Pump full to the top and then see the millage . As a habit I keep topping up every week so my tank /glass is always 1/2 full or empty in that way you look at it 🙂

I've seen BJ's been advertised as doing 12-13 Km on Colombo and over 20km outstation - maybe they used wind power such as mast and a sail.

I hear there are some stories about a fuel hike also on the way ....

iRage · May 24, 2021

On 5/23/2021 at 5:00 PM, Dhaham said:

Thanks for your comment.

I think, another method is, we can fill up to the lid.

Anyway, it has to be ensured that the car is on a flat, horizontal surface.

Sometimes I have heard some people are saying, "that Vitz is doing 15-16 in Colombo and 20 in out stations".

Also heard something like "My Alto / Mira is doing 18 in city and 24 in long trips".

Most of this kind of words are due to not knowing how to calculate the fuel deficiency correctly. Otherwise, they are telling lies.

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The other issue is people do not realise the difference between a tank average and a trip average (for the most part because their Vitzies and Premios have it written in Japanese and they have no idea what it actually says)

Yes..the car needs to be flat..but for the most part petrol sheds by default do remain levelled. Filling ot the brim can be tricky as sometimes the bleeder valve/line circulates the fuel so you do not really know if it is actually filled to the brim or not. Besides, most manufacturers do not recommend you fill right to the cap because of, in the past, pressure in the tank reasons, and the more modern day problem is the calculation of the fuel stats itself.

Also, petrol changes its density based on ambient tempreture (and my mom said I wouldn't learn anything from Top Gear). So you should try to pump around the same time of day (if you are being really OCD about it )

MaleCortana · May 25, 2021

On 5/24/2021 at 2:28 AM, iRage said:

Also, petrol changes its density based on ambient tempreture (and my mom said I wouldn't learn anything from Top Gear). So you should try to pump around the same time of day (if you are being really OCD about it )

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Pump at night for more petrol for your hard earned monies I say!