How do I work out MPG?

hardpressed

If my car has done 300 miles on 25 litres of petrol how many MPG is that. I can't even begin to think how to work it out!

Derivative

1 imperial gallon = 4.55 litres.

So 25 litres = 5.5 gallons.

300/5.5 = 55mpg. Not bad for a car.

KierNet

I use http://www.torquecars.com/tools/uk-mpg-calculator.php

Hintza

To be accurate you need to fill the tank to the brim and note mileage. When you next fill up brim again note mileage and amount of petrol used.

OP you are just guessing (OK might not be far away) that you have used 25l.

cyclonebri1

hardpressed wrote: »

If my car has done 300 miles on 25 litres of petrol how many MPG is that. I can't even begin to think how to work it out!

Gets asked and argued over many times on here, but as above the greater mileage and going brim to brim will give the most accurate results. The car fuel computer if you have one is accurate enough for most folks although only as good as the car speedo/mileometer ie, could be 10% optomistic

C_Mababejive

Hintza wrote: »

To be accurate you need to fill the tank to the brim and note mileage. When you next fill up brim again note mileage and amount of petrol used.

OP you are just guessing (OK might not be far away) that you have used 25l.

I think it would be accurate over a longer period if you always filled up with exactly the same amount as measured at the pump in litres.then clocked the mileage?? Wouldnt have to be full..

kaya

the brim to brim method is NOT accurate, fuel is drawn from under the ground into your fuel tank, like most liquids as it heats up it expands, which makes your reading inaccurate, also you would have to run your car until it starved of fuel to know the tank was empty both times and allow for the volume expansion due to ambient temperature, the only accurate way would be to weigh your fuel each time, which is totally impractical, the weight of fuel does not change with temperature like the volume does.

Derivative

Over say, 30 degrees C, petrol doesn't really expand enough for that to matter.
You'll be within enough to get a decent estimate. Your driving style will likely influence the measurement more than being "exactly right" on the brim to brim method.

This is because when you look at an entire tank, you're actually taking five or six MPG measurements and averaging them.

Hammyman

hardpressed wrote: »

If my car has done 300 miles on 25 litres of petrol how many MPG is that. I can't even begin to think how to work it out!

Unbelievable. Even my 9 year old son can work that out and he's autistic.

Lets get it right. Its not that you can't work it out BUT YOU ARE TOO BONE IDLE TO.

I wish people on these forums would stop pampering to lazy people like the OP and giving them the answer.

Hintza

kaya wrote: »

the brim to brim method is NOT accurate, fuel is drawn from under the ground into your fuel tank, like most liquids as it heats up it expands, which makes your reading inaccurate, also you would have to run your car until it starved of fuel to know the tank was empty both times and allow for the volume expansion due to ambient temperature, the only accurate way would be to weigh your fuel each time, which is totally impractical, the weight of fuel does not change with temperature like the volume does.

Your point being?

It is about as accurate as the lay person can get without fannying around.

asbokid

kaya wrote: »

the brim to brim method is NOT accurate, fuel is drawn from under the ground into your fuel tank, like most liquids as it heats up it expands, which makes your reading inaccurate, also you would have to run your car until it starved of fuel to know the tank was empty both times and allow for the volume expansion due to ambient temperature, the only accurate way would be to weigh your fuel each time, which is totally impractical, the weight of fuel does not change with temperature like the volume does.

The volumetric thermal expansion coefficient of petroleum will have only a small impact on the calculation of fuel efficiency of a family car.

The thermal expansion coefficient for petrol is (9.5*10^-4/C)

An average petrol car has a fuel tank of 60 litre capacity and a fuel efficiency of 35mpg (12.39kpl)

If the temperature of the fuel rises from 15C at the time of purchase to 25C then the 60 litres of petrol will expand in volume by:

(9.5*10^-4/C)(60)(25-15) = 0.570 litres

That extra volume of petrol could power the vehicle for another 7.06 kilometres or 4.39 miles, but instead it overflows the tank and is wasted on the floor.

However there are other environmental variables to factor into our calculations such as barometric pressure.