MPG Maths help!

kingjez · 26th March 2008, 22:13

Hi everyone,

I know there are loads of threads about MPG on here so apologies if this has been covered before, but I've got a maths problem related to MPG:

I've set my fuel economy gauge to auto-reset when I refill the tank. The last time I refilled the tank the gauge was reading just over 40mpg. Given that on my previous refill, I had filled the tank completely (or at least until the pump had clicked off a couple of times), I figure that the trip A counter is pretty representative of the number of miles I've done for the quantity of fuel I put in the tank. So having put in 42.16 Litres, and recorded 344.0 miles on Trip A, I set about working out what MPG that is. The answer I got was 30.9. This seems a long way short of the 40ish the car told me. What have I done wrong? Has anyone else tried doing this comparison?

sallytraffic · 26th March 2008, 22:15

Quote:

Originally Posted by kingjez

What have I done wrong? Has anyone else tried doing this comparison?

First question: Have you been filling at exactly the same pump ? I ask because some click off a lot earlier than others and the difference can make major changes to the calculations.

Jack The Lad. · 26th March 2008, 22:26

I have had a long day, kicking ass with the building contractors on site, but I make it --37.109 miles per imperial gallon.

pjr · 26th March 2008, 22:50

Jack The Lad is correct.

42.16 litres divided by 4.546 litres per gallon gives 9.27 gallons

344 miles divided by 9.27 gallons gives 37.109 mpg.

The trip computer does usually read slightly high, so you're not far off. Although over 1 tank of petrol, and possibly from a different pump (as Sallytraffic mentioned) you might get slightly different answers.

Only sure way is do calculate it manually over several tanks.

sallytraffic · 26th March 2008, 22:51

Quote:

Originally Posted by Jack The Lad.

I have had a long day, kicking ass with the building contractors on site, but I make it --37.109 miles per imperial gallon.

Yes, I got pretty much the same:

42.16 l is about 9.27 gallons.

344 / 9.27 is about 37.1 mpg

This fits reasonably well as the real amount compared to the car figure, 3 or 4 mpg less than indicated. That seems quite usual.

sallytraffic · 26th March 2008, 22:52

Snap

Jack The Lad. · 26th March 2008, 23:06

I was going to take it to 8 decimal points ST, but then I thought -------------S-d it

sallytraffic · 26th March 2008, 23:08

Quote:

Originally Posted by Jack The Lad.

I was going to take it to 8 decimal points ST, but then I thought -------------S-d it

Answer to no more accuracy than the values used in the calculation, please

Jack The Lad. · 26th March 2008, 23:14

You remind me of my teacher at architectural college.
We used slide rules in those days.

sallytraffic · 27th March 2008, 00:04

Quote:

Originally Posted by Jack The Lad.

You remind me of my teacher at architectural college.
We used slide rules in those days.

You were lucky.

I used an abacus.

Jack The Lad. · 27th March 2008, 00:13

Kremmen · 27th March 2008, 07:11

Considering the state of electronics these days.....

MPG is wrong

MPH is wrong

Not too impressive is it ?

It is annoying when my Talex is reading 70 mph, the Citroen C4 next to me is reading 70 mph, but I'm reading 76 mph !

kingjez · 27th March 2008, 10:15

I've spotted my mistake - I was using US liquid gallons, not imperial gallons!! That's what comes from using google to do the conversion!! Multiply my answer by 1.2 and I get roughly what you guys get - phew!

I shall now start to average over lots of tanks, and see how it works out!

Thanks everyone!

GHGH · 27th March 2008, 10:45

After 13000 miles, my real consumption is 9% higher than stated, and this varied from filling to filling depending on how much you top up the tank, driving conditions, what do I know....

dylang652 · 27th March 2008, 11:22

Quote:

Originally Posted by GHGH

After 13000 miles, my real consumption is 9% higher than stated, and this varied from filling to filling depending on how much you top up the tank, driving conditions, what do I know....

My real consumption is only 2% higher than stated after 10000 miles (49.6 compared to the 50.8 stated by the computer)

And having 'Exceled' my fuel consumption since new (sorry... but I am a computer geek!)

The figures per tank are very close with the computer over-reading 11 times, under-reading 8 times and spot on 3 times

So I guess I am lucky and have a good set of sensors for the computer to work with

GHGH · 27th March 2008, 11:32

Yep, I've noticed that people reports varying accuracy.
Is there any difference between the 2.2 and 1.8 or is it pure chance?
Anyway it's really no big deal, but it's funny hor variable these figures are - not only for the civic.

Unknownsoldier · 27th March 2008, 11:39

The amount of variables to take into consideration is immense:

Read-out innacuracy,
Different pumps,
Temperature of tank, the fuel itself and the external petrol tank,
Atmospheric pressure,
Fuel type,
Country filling in,
Engine set-up,
Route driven and style of driving.

ATB,
Tom

26th March 2008, 22:13	#1 (permalink)
kingjez Valve Cap Join Date: 4th February 2008 Location: London, UK Posts: 8 Thanks: 0 Thanked 0 Times in 0 Posts	Maths help! Hi everyone, I know there are loads of threads about MPG on here so apologies if this has been covered before, but I've got a maths problem related to MPG: I've set my fuel economy gauge to auto-reset when I refill the tank. The last time I refilled the tank the gauge was reading just over 40mpg. Given that on my previous refill, I had filled the tank completely (or at least until the pump had clicked off a couple of times), I figure that the trip A counter is pretty representative of the number of miles I've done for the quantity of fuel I put in the tank. So having put in 42.16 Litres, and recorded 344.0 miles on Trip A, I set about working out what MPG that is. The answer I got was 30.9. This seems a long way short of the 40ish the car told me. What have I done wrong? Has anyone else tried doing this comparison?

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Jack The Lad. Supporter Rocketship door handle Join Date: 28th August 2007 Location: ENGLAND Posts: 1,396 Thanks: 0 Thanked 8 Times in 8 Posts	I have had a long day, kicking ass with the building contractors on site, but I make it --37.109 miles per imperial gallon.

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pjr Supporter Triangular Exhaust Join Date: 24th March 2007 Location: Scotland Posts: 395 Thanks: 7 Thanked 4 Times in 4 Posts	Jack The Lad is correct. 42.16 litres divided by 4.546 litres per gallon gives 9.27 gallons 344 miles divided by 9.27 gallons gives 37.109 mpg. The trip computer does usually read slightly high, so you're not far off. Although over 1 tank of petrol, and possibly from a different pump (as Sallytraffic mentioned) you might get slightly different answers. Only sure way is do calculate it manually over several tanks.

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Jack The Lad. Supporter Rocketship door handle Join Date: 28th August 2007 Location: ENGLAND Posts: 1,396 Thanks: 0 Thanked 8 Times in 8 Posts	You remind me of my teacher at architectural college. We used slide rules in those days.

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27th March 2008, 10:15	#13 (permalink)
kingjez Valve Cap Join Date: 4th February 2008 Location: London, UK Posts: 8 Thanks: 0 Thanked 0 Times in 0 Posts	I've spotted my mistake - I was using US liquid gallons, not imperial gallons!! That's what comes from using google to do the conversion!! Multiply my answer by 1.2 and I get roughly what you guys get - phew! I shall now start to average over lots of tanks, and see how it works out! Thanks everyone!

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GHGH TANSTAAFL! Magic Rear Seat Join Date: 13th March 2007 Location: Trondheim, Norway Posts: 920 Thanks: 0 Thanked 0 Times in 0 Posts	After 13000 miles, my real consumption is 9% higher than stated, and this varied from filling to filling depending on how much you top up the tank, driving conditions, what do I know....

27th March 2008, 11:32	#16 (permalink)
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27th March 2008, 11:39	#17 (permalink)
Unknownsoldier Smart Ase Civinfo guru* Join Date: 22nd September 2007 Location: Kent, UK Posts: 2,079 Thanks: 28 Thanked 10 Times in 10 Posts	The amount of variables to take into consideration is immense: Read-out innacuracy, Different pumps, Temperature of tank, the fuel itself and the external petrol tank, Atmospheric pressure, Fuel type, Country filling in, Engine set-up, Route driven and style of driving. ATB, Tom