How do I work out MPG?

Garetha

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!

Perhaps it's time we metricated ourselves, understanding Litres per 100k.

asbokid

Originally Posted by 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!

25 litres = 5.5 Imperial gallons (25/4.546)

300 miles / 5.5 = 54.55 mpg.. impressive.

reeac

asbokid wrote: »

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


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.

You won't lose all of the extra volume as it will take time to warm up and expand and during that time you will be consuming fuel and thus creating tank space for expansion. Depens upon how far you drive after filling up.

A second point is that energy content of fuel is proportional to weight not volume so your warm, expanded fuel will give lower mpg than will cold fuel - not a big effect though.

AHAR

kaya wrote: »

...also you would have to run your car until it starved of fuel to know the tank was empty both times

:idea:
Or alternatively refill to the brim again before it's empty and use the volume displayed on the petrol pump as the figure to use in your calculation rather than the tank capacity listed in your handbook!

Flyboy152

reeac wrote: »

You won't lose all of the extra volume as it will take time to warm up and expand and during that time you will be consuming fuel and thus creating tank space for expansion. Depens upon how far you drive after filling up.

A second point is that energy content of fuel is proportional to weight not volume so your warm, expanded fuel will give lower mpg than will cold fuel - not a big effect though.

As well as reducing the effect of the overall co-efficient.

Flyboy152

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.

There would be no need to empty the tank. All one would need to do, is use a quantity of fuel, replace it, then calculate the distance travelled; quite simple really.

asbokid

reeac wrote: »

A second point is that energy content of fuel is proportional to weight not volume so your warm, expanded fuel will give lower mpg than will cold fuel - not a big effect though.

The energy density of petroleum can be measured either by volume (MJ/l or Wh/l) or by mass (MJ/kg or Wh/kg).

It is possible to infer the gravimetric energy density of petroleum from its volume, as a function of temperature.

In fact, the calibration of a volumetric flow-meter in a petrol pump is performed using a temperature compensation function.

C_Mababejive

Flyboy152 wrote: »

There would be no need to empty the tank. All one would need to do, is use a quantity of fuel, replace it, then calculate the distance travelled; quite simple really.

But how would you know when you had used that quantity of fuel?

asbokid

cyclonebri1 wrote: »

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, i.e. could be 10% optimistic

According to the Mazda forum, the fuel computer serves as a very poor gauge of fuel efficiency.

The calculation is a crude estimate based on the flow rating of the fuel injector, a count of the injection pulses and the duration of the pulse. However, as the injectors age, they can clog, restricting the amount of fuel that flows through them. This can impact on the accuracy of the calculation.

scragend

C_Mababejive wrote: »

But how would you know when you had used that quantity of fuel?

When you refilled.

If you start with a full tank, then use some, then fill the tank again, the amount of fuel it takes to refill the tank is the amount you have used. You then use that amount, and the mileage you have done, to work out the MPG.

This can be any amount of fuel (up to a maximum of what your tank holds obviously). You don't have to decide beforehand what the "quantity of fuel" is going to be.