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Calculating MPG

Blog Date - 23 December 2017

Sharon asked me to explain how to calculate MPG because she gets asked quite often on Facebook. So here we go...

You need to know how much fuel you have used. The simplest way to do this is fill your tank up. Once this is done zero the trip meter on your motorcycle. This will be different from model to model so you'll need to consult your manual as to how to do this. 

You will now have a full tank of fuel and the trip meter set to zero.

Then ride your bike. 

When you're ready to refuel fill the tank once again. Get a receipt from the petrol station that shows how much fuel in litres you've put into the tank.

Remember you set the trip meter to zero when you first filled the tank? Now the trip meter will show how many miles you've covered since that fill up. Ideally if you have a pen you can write this on the receipt - otherwise you'll have to memorise the distance. Once you've noted the trip meter's reading set it to zero once again.

So you know how many miles you have ridden since the first fill up and how much fuel you've used since the first fill up.

Maths!

Oddly the UK is supposed to be a metric country but most people talk about "Miles Per Gallon" - it should be Kilometres Per Litre or even Litres Per 100 Kilometres but let us not worry about that. Well...except your receipt will be in Litres not Gallons. It is worth noting this is for UK imperial gallons - American gallons are slightly smaller.

To work out how many gallons of fuel you put into the motorcycle take the litres from your receipt and divide it by 4.54 (you'll need a calculator - I use the one on my phone).

So if your receipt shows you put in 10.35 litres then divide 10.35 by 4.54. This will give you 2.729.......and lots of decimal points. This means you put in 2.73 imperial gallons of fuel. 

We're after MILES per GALLON. We have worked out the gallons and we noted the miles. All we do now is divide the miles we covered by the gallons we have worked out. 

So you noted you'd covered for example 186 miles. You have worked out you put in 2.73 gallons of fuel. Divide 186 by 2.73 and we get 68.131.....and lots of decimal points. You can round this to 68, which means you have done 68mpg.

So

Divide litres by 4.54 to get gallons.
Divide miles by gallons to get MPG.

A few more examples.

214 miles and 9.87 litres of fuel
9.87 divided by 4.54 gives 2.17........ gallons
214 divided by 2.17 gives 98.6.........MPG

9.65 litres and 143 miles
9.65 divided by 4.54 gives 2.12.........gallons
143 divided by 2.12 gives 67.4..........MPG

an old trip meter on a motorcycle


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Reader's Comments

GJ said :-
Rather than testing my arithmetic skills.

I just use apps on my Iphone called Fuelly Gas Tracker and Fuel Monitor.

You enter Odometer Reading, Price Of Fuel, Price Per Litre and they give you the mpg.
I log every fill up.

I have a record of mpg for each fill up after the initial setup one.
28/12/2017 08:39:27 UTC
Ren - The Ed said :-
I've had a few comments on Facebook regarding using various apps and websites. It is of course the most logical way to do it. None the less it is a good thing to know HOW to calculate it, to understand is to know.
28/12/2017 11:44:17 UTC
Ian Soady said :-
Never bothered with the bike as I'm happy to accept whatever I get - but interestingly, rough checks show that I had more or less the same 50-55 mpg from my 850 Commando, 955i Hinckley Tiger and V-Strom 650. More to do with riding style than anything else. Fill the tank on leaving the overnight stop and you're good for a couple of hundred miles easily.

But with the car I religiously enter every litre of fuel with the speedo reading into a spreadsheet - I now have 60,000 miles worth. It also calculates running cost / mile, as well as CO2 emissions (quite scary). I only have to enter fuel quantity, price and mileage reading for each fill-up.

Checking on anything less that several tankfuls will not give you a realistic reading - my spot readings vary by 30% or so depending on usage.
28/12/2017 11:55:12 UTC
Borsuk said :-
I use Fuelly for my car and my bike. Keeping track of my mpg isn't just for my wallet's sake. If my mpg starts to decrease for no apparent reason then it could be an indication that my engine isn't running right. Though I have never managed to get the mpg they quote in the book for any of my cars I do get quite a good value so I am not driving like a total nutter anymore. The 125 is averaging about 105 mpg which is suiting me fine. Wish I could get that out of the car as well, which a Suzuki team have managed to exceed according to some reports I read a few years ago. These guys must have Iron control of their feet and prescient abilities to avoid braking unnecessarily.
29/12/2017 11:31:24 UTC
Ren - The Ed said :-
Fuel economy is important to me because I cover around 20k per year. The 125 makes a huge difference to my fuel bill but I often wonder how much of that is lost in parts.

Borsuk - that Suzuki team would have used a flat large oval track and maintained an optimum speed for that engine. No stops, no lights, no roundabouts, no hills, no corners....
29/12/2017 12:29:12 UTC
 

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