Miles per Gallon: Metric to Imperial

mrtom

Hi perhaps this is the forum to help me. Recently bought a Fiesta car and am trying to establish the MPG. I do know that 20.3 L gave 225Km. What is my MPG?

Fiskar

31.49 Mpg

hivizman

More generally, to convert a fuel consumption figure expressed in km/l in terms of MPG, note that 1 gallon = 4.5461 litres and 1 km = 0.6214 miles, so 1 km/l = 4.5461 x 0.6214 MPG = 2.825 MPG.

Fuel consumption is often stated in litres per 100 km (l/100km). In the example given, we have 20.3 litres consumed to drive 225 km. We can express this as consumption of 11.084 km/l, or as 9.022 l/100km.

To convert the km/l figure, simply multiply by 2.825 to get 31.31 MPG (same basically as Fiskar allowing for approximations).

To convert the l/100km figure, divide this into 282.5 to get 31.31 MPG.

mrtom

Gentlemen, your answer impresses me more than the car!

Fiskar

hivizman wrote: »

More generally, to convert a fuel consumption figure expressed in km/l in terms of MPG, note that 1 gallon = 4.5461 litres and 1 km = 0.6214 miles, so 1 km/l = 4.5461 x 0.6214 MPG = 2.825 MPG.

Fuel consumption is often stated in litres per 100 km (l/100km). In the example given, we have 20.3 litres consumed to drive 225 km. We can express this as consumption of 11.084 km/l, or as 9.022 l/100km.

To convert the km/l figure, simply multiply by 2.825 to get 31.31 MPG (same basically as Fiskar allowing for approximations).

To convert the l/100km figure, divide this into 282.5 to get 31.31 MPG.

Only 0.32% out on the approximation though, I use a simpler method

Basically I believe the OP was more interested in the baseline figure of MPG for the car. IMO the MPG figure for that car is quite bad so it must be city driving! A Yaris would / could not be as bad!

MathsManiac

hivizman wrote: »

More generally, to convert a fuel consumption figure expressed in km/l in terms of MPG, note that 1 gallon = 4.5461 litres and 1 km = 0.6214 miles, so 1 km/l = 4.5461 x 0.6214 MPG = 2.825 MPG.

Fuel consumption is often stated in litres per 100 km (l/100km). In the example given, we have 20.3 litres consumed to drive 225 km. We can express this as consumption of 11.084 km/l, or as 9.022 l/100km.

To convert the km/l figure, simply multiply by 2.825 to get 31.31 MPG (same basically as Fiskar allowing for approximations).

To convert the l/100km figure, divide this into 282.5 to get 31.31 MPG.

If you're doing it in your head, as you might be while driving, for example, then dividing the number into 280 is a good approximation for switching between litres per 100km and MPG. (Works in both directions, obviously.)

hivizman

Fiskar wrote: »

Only 0.32% out on the approximation though, I use a simpler method

Care to share it?

Fiskar wrote: »

Basically I believe the OP was more interested in the baseline figure of MPG for the car. IMO the MPG figure for that car is quite bad so it must be city driving! A Yaris would / could not be as bad!

Mathematicians have a habit of looking for more general approaches to problems, so a general formula is often considered more useful than an answer to a specific question.

In practice, like MathsManiac, I would use approximations. I'd probably work out the MPG by converting a rate per litre to a rate per gallon by multiplying by 9 and dividing by 2, and kilometres into miles by multiplying by 5 and dividing by 8. So a consumption of 20.3 litres to drive 225 kilometres would be roughly a rate of 11 k/l, which I'd express as 11x9/2 = 49.5, say 50 k/gallon, which is 31.25MPG, and because I'm using approximations I'd ignore the decimals and call this a consumption of around 30 MPG. As you note, this is not a very impressive fuel consumption, but you need to know the mix between different modes of driving to give a definite opinion.

mrtom

Good afternoon all. I'm back with further data for you to crunch. I'm back from a run to Sligo and filled my tank twice: This will give me an idea of my fuel consumption with motorway driving.

Distance :312 km Fuel consumption: 20.5L
Distance :234 km- Fuel consumption: 15.8L

Yes, I appreciate “Give a man a fish feed him for a day, give him a rod.....” I have tried they various computations suggested however my pass maths failed me and I was getting ridiculous numbers!

Agent_47

mrtom wrote: »

Good afternoon all. I'm back with further data for you to crunch. I'm back from a run to Sligo and filled my tank twice: This will give me an idea of my fuel consumption with motorway driving.

Distance :312 km Fuel consumption: 20.5L
Distance :234 km- Fuel consumption: 15.8L

Yes, I appreciate “Give a man a fish feed him for a day, give him a rod.....” I have tried they various computations suggested however my pass maths failed me and I was getting ridiculous numbers!

Ist: 43.24 Mpg

2nd: 42.08 Mpg

You're getting better at the driving

HiVizMan will indicate 42.99Mpg in case 1 and 41.836 Mpg in case 2.

Fiskar

Agent_47 wrote: »

Ist: 43.24 Mpg

2nd: 42.08 Mpg

You're getting better at the driving

HiVizMan will indicate 42.99Mpg in case 1 and 41.836 Mpg in case 2.

I concur with these figures

Must have changed the car.

Fiskar

hivizman wrote: »

Care to share it?



Mathematicians have a habit of looking for more general approaches to problems, so a general formula is often considered more useful than an answer to a specific question.

In practice, like MathsManiac, I would use approximations. I'd probably work out the MPG by converting a rate per litre to a rate per gallon by multiplying by 9 and dividing by 2, and kilometres into miles by multiplying by 5 and dividing by 8. So a consumption of 20.3 litres to drive 225 kilometres would be roughly a rate of 11 k/l, which I'd express as 11x9/2 = 49.5, say 50 k/gallon, which is 31.25MPG, and because I'm using approximations I'd ignore the decimals and call this a consumption of around 30 MPG. As you note, this is not a very impressive fuel consumption, but you need to know the mix between different modes of driving to give a definite opinion.

This variant allows me work out a result in my head without the use of computers.

1) Convert Km to miles (divide Km by 8 and * 5) = 225/8 * 5 = 140.6 miles

2) Convert litres to gallons (divide no. of litres used by 4.546) = 20.3/4.546 = 4.465 gallons

3) To get MPG divide 140.6 by 4.465 = 31.49 Mpg

Simples