How Is Average Mpg Calculated?

[Peter_H]

Hi,

I'm sorry if this question is going over old ground, I've found various topics on here related to average mpg, but none as far as I can see go into exactly how it is calculated. If I'm wrong, please just cut an ignorant newbie some slack.

Clearly there is a "spot" mpg value, but how does the computer turn it into an average? How far back in time does it look at these spot values to get the average?

I just mention this because I picked up an iQ last weekend, started looking at this number and found it was around 40mpg, and couldn't help being underwhelmed. Especially seeing some of the numbers people talk about on this forum - sixty-something mpg etc.

Since I got the car I've driven it almost every day, but only quite short distances. Over the course of the week I've seen the average mpg creep up such that the computer now reads around 45mpg. ......which seems to suggest that the computer works out the average over a number of weeks? ...or does it?

Does anyone know for sure?

Thanks,

Pete

[MrSandman]

Hi,

I'm sorry if this question is going over old ground, I've found various topics on here related to average mpg, but none as far as I can see go into exactly how it is calculated. If I'm wrong, please just cut an ignorant newbie some slack.

Clearly there is a "spot" mpg value, but how does the computer turn it into an average? How far back in time does it look at these spot values to get the average?

I just mention this because I picked up an iQ last weekend, started looking at this number and found it was around 40mpg, and couldn't help being underwhelmed. Especially seeing some of the numbers people talk about on this forum - sixty-something mpg etc.

Since I got the car I've driven it almost every day, but only quite short distances. Over the course of the week I've seen the average mpg creep up such that the computer now reads around 45mpg. ......which seems to suggest that the computer works out the average over a number of weeks? ...or does it?

Does anyone know for sure?

Thanks,

Pete

Hi pete

Well I am in same situation as you. I have just bought a 08 Tr Avensis a month ago, now on display it read 51mpg, after a couple of weeks and about £200 worth of fuel later it read 48mpg. To get the most accurate reading of mpg tank up to the brim and then drive till the reserve light lights up and then divide the mileage by 55litre or convert 55 ltrs to gallongs and then divide by gallons. Hopefully you should get a much more accurate reading.

I reset my computer to see how accurate it is,,, I am currently getting around 47mpg 50/50,, and around 38-40 short runs local, and if on a motorway its about 60+mpg in 6th gear at approx 70mph. It depends on the persons driving style, i always try to change gear at 1800-2200 revs also I use cruise control as much as I can. Just recently I put ion £10 worth of diesel from reserve and drove it 50/50 till reserve again and it gave me 83 miles before reserve light lit up, thats 83miles on 1.56 gallons (£10 diesel).

Hope that helps lol

[MattIQ]

As far as I know, the Average MPG is calculated from the last fill up, since it resets back to 0 when you fill the car with fuel.

I think it's unlikely you'll see 60+mpg with an IQ3, most people with those figures tend to have a 1.0 litre IQ/IQ2.

Oh, and in relation to the second post, the fuel tank is only 32 litres on the IQ.

[Peter_H]

it resets back to 0 when you fill the car with fuel.

So do you reckon the computer keeps a log of how much fuel is in the tank, and at ignition says:

if ("current fuel in tank" > "fuel in tank the last time I looked") then reset average to 0

I suppose that makes sense, presumably one of the last things the dealer did before I picked the car up was to fill the tank, thereby resetting it to zero. And the first few miles of my first journey was crawling through Saturday shopping traffic to get home, which presumably will give a poor mpg to start with. Since then I've tended to drive either early morning or late evening, when traffic would have been less stop-start and therefore I've got a better average.

Does average mph work the same way?

Sandman, I'm impressed that you went to the trouble to calculate all these numbers. Be interested to know how accurate you think your computer is - I've read through quite a few posts on the iQ forum and from what I've seen the consensus appears to be that on the iQ the number is not 100% accurate but is there or thereabouts. For me, that's probably as much as I can be bothered to know.

(I do find it a bit disconcerting that I've probably done 90ish miles since I got the car, which at 45mpg equates to 2ish gallons, out of a 7ish-gallon tank, and the fuel gauge still says "full". But I have read that other people on here have seen funnies with the accuracy of the fuel gauge given the flat tank.)

[Cyker]

I thought it just keeps adding all the spot mpg readings together and then divides them to get the average!

The 'mean' average as mathematics people would say :D

[Peter_H]

I thought it just keeps adding all the spot mpg readings together and then divides them to get the average!

The 'mean' average as mathematics people would say :D

Well, my car was first registered in Jan 2010. Are you suggesting that the computer calculates the average based on all the spot mpg readings since then? lol

[Cyker]

Well given people are saying it resets when you refuel I'd expect it resets itself then.

I'm only guessing as my poor diesel Yaris doesn't get one of these fancy trip computers :( (Not like the smug petrol Yaris owners! )

[s44gtw]

On an IQ it is not when you 'fill up' that it resets the 'AVG mpg' on the screen.

Its supposed to be when you put in over 10 litres of petrol.

But,

If you have gone 100 mile since the last fill up when the computer reset after driving off

& it is now showing AVG 49 mpg,

You might pull in & put in 11 litres, zero your trips & pull away & the average will not reset.

The car can be sitting at an angle or what ever & it might of needed 15 litres to reset.

(there can be quite a bit difference on how much petrol you can get up the tube, till you see it)

I never had Average available to anything like being near correct from fill up to fill up untill 1000 miles or so was done from new.

Now its easy to put in over 10 litres of juice, leave the garage and get the average showing way up to 65 mpg plus,

before settling to nearer the true Miles per gallon from that last fueling.

IME it then hardly shows any difference over the next while, it changes very very slowly,in an iq3 i think you will need to try very hard to achieve a good regular MPG, let alone get movement in the AVG display.

The next time reset showing around the 48,50, 52 MPG or what ever is only 'nearish' to what you have actually done.

IMO only brimming & checking the miles covered & dividing the litres used by 4.546 gives a closer actual MPG Average for that stint.

george

[Peter_H]

Its supposed to be when you put in over 10 litres of petrol.

Thanks very much George, that sounds pretty definitive. That there are a number of caveats to this isn't really surprising. As I said in an earlier post on this topic I'm already seeing readings I don't particularly believe from the fuel gauge.

Pete

[thewildblue]

Well I am in same situation as you. I have just bought a 08 Tr Avensis a month ago, now on display it read 51mpg, after a couple of weeks and about £200 worth of fuel later it read 48mpg. To get the most accurate reading of mpg tank up to the brim and then drive till the reserve light lights up and then divide the mileage by 55litre or convert 55 ltrs to gallongs and then divide by gallons. Hopefully you should get a much more accurate reading.

Nearly correct, you replace 55 litres with what you actually put in rather than assuming thats what you have used. I know for a fact that in most vehicles you can always squeeze in more than the specs usually allow for. They dont usually take into account the filler neck.

I usually take my measurements on the first click when filling up, now this can be dependant on where you fill up, but generally I try and use the same garage and pump whenever I can.

The trip computer on the IQ isnt that bad Ive had worse with other cars, where they are out of whack everytime. Sometimes the IQ is spot on. :-)

[MrSandman]

Well I am in same situation as you. I have just bought a 08 Tr Avensis a month ago, now on display it read 51mpg, after a couple of weeks and about £200 worth of fuel later it read 48mpg. To get the most accurate reading of mpg tank up to the brim and then drive till the reserve light lights up and then divide the mileage by 55litre or convert 55 ltrs to gallongs and then divide by gallons. Hopefully you should get a much more accurate reading.

Nearly correct, you replace 55 litres with what you actually put in rather than assuming thats what you have used. I know for a fact that in most vehicles you can always squeeze in more than the specs usually allow for. They dont usually take into account the filler neck.

I usually take my measurements on the first click when filling up, now this can be dependant on where you fill up, but generally I try and use the same garage and pump whenever I can.

The trip computer on the IQ isnt that bad Ive had worse with other cars, where they are out of whack everytime. Sometimes the IQ is spot on. :-)

I stand corrected...yes that is the best way and more accurate .

[Daveyonthemove]

Well.... to throw another inaccuracy into the mix, here is my thoughts on it.

If you fill it to the first 'click' and then stop, you cannot be sure to have stopped at the same level each time. If the flow of fuel is going faster than the last time you filled up, it could splash up sooner and mean you have less fuel than other fill ups.

The only way to ensure you know EXACTLY how much fuel you use, is to fill to the brim. That way there is no possible way of getting any more fuel in there, and you know you haven't got less fuel either.

Then you know that you were full to the brim last time, you covered XXX amount of miles, and needed XX amount of fuel to fill it back to the brim. Then your maths should be spot on.

On the topic of average MPG calculations, i don't know how the car does it, but when i fill my car up using the above method and compare it to the computer, the MPG readings are close to within 0.5 mpg.

Shame the 'Range' function isn't as accurate

[marcandsebe]

I have brought a iPhone app called Fuel Calc by Corbenic Consulting. It works out that my 2.5 Nissan Navara is doing nearly 35 mpg and the 1.3 iQ is doing over 36 mpg. Not the result I was expecting. They are both auto. I do thrash the little iQ around though.