Calculating MPG without emptying the tank?

IanAird · Post by **IanAird** » 27 May 2005, 21:11

MPG is not too hard to measure accurately - fill the fuel tank to brim full - CAREFULLY - reset the odometer to zero - pay for the fuel - drive away!
At ANY time thereafter you can do an mpg check - go to a fuel station, fill the fuel tank to brim full - CAREFULLY AGAIN - and this time Note the amount of fuel in litres or gallons to brim the tank - then note the odometer mileage reading BEFORE you reset it to zero.
If buying in gallons, UK or USA, then the mileage divided by gallons = miles per gallon.
If buying in Litres, Mileage divided by litres = mile per litre.
To convert to miles per UK gallon simply multiply the miles per litre by 4.54609 as there are about 4.54609 litres to the UK gallon.
If you are in the USA, a US gallon is only about 3.785 litres, so multiply the miles per litre figure by 3.785 to get miles to the US gallon.
OK? [:)]

VisaGTi16v · Post by **VisaGTi16v** » 27 May 2005, 21:29

Yep, something I do quite often to see if I can get 20 or more mpg

This however is something my friend cannot accept. He says how can it be accurate when you may use different pumps that cut at different amounts etc. This may be true but will only change the amount by a few ml. Instead he goes on about the accurary of his petrol gauge etc sigh. Needs a good slap!

James.UK · Post by **James.UK** » 27 May 2005, 22:31

I use the same method, but then go to this website for results [:D]
http://javascript.internet.com/equivale ... llon2.html
.

jeremy · Post by **jeremy** » 28 May 2005, 00:04

The secret is to carry on filling after the pump has cut out (sometimes best part of a gallon) and stop when you can actually see the fuel in the filler neck. This is best done when you are about to drive a distance to burn off some of the fuel straight away.
Of course if your mileomter overeads by 5% your resulting figure will be innaccurate as well.
Having filled the car its not a bad idea to have a really good look and smell underneath just in case something (like a vent pipe) is leaking.
jeremy

adamskibx · Post by **adamskibx** » 28 May 2005, 00:38

I use this method and just do a conversion on my phone calculator. I just remember that my Dyane fuel tank was 25 litres which is also 5.4 gallons as stated in the handbook and do a division which comes out at 4.68 or whatever then multiply miles per litre by that number. Strange the way i remember that more than the actual conversion factor.

madasafish · Post by **madasafish** » 28 May 2005, 01:43

I fill brim to brim and record my purchases and consumption so I can calculate over the past year/ 2years -- sad!.
Filling as suggested previously only works IF the petrol station ground is level. If it slopes in such a way as to leave an air pocket in the tank your results may be out by up to 2-3 litres. And if the next garage slopes the other way:-)..
Long term figures are the only true measure - all else depends on too may short term variables: like your driving style/ city vs motorway driving etc.
( I found my 35 year old records for my 1929 Riley 9 - the first car I ever owned. NOT new of course- very secondhand but I was the second owner !. It averaged around 24mpg. Fuel injection? Wahatsthat? A fuel pump? Whatsthat? (gravity feed). Water pump? Whatsthat? (Thermosyphon). Syncromesh? and so on.
Aplogies for going offtopic!

Kowalski · Post by **Kowalski** » 28 May 2005, 05:35

I've got 18,424.40 miles of continuous fuel economy figures for my older Xantia and 27,712.00 of continuous figures for the new one.
The old car averaged 43.05mpg over its mileage where as the new one averages 46.11. The new car is a bit more economical than the old one but route and driving style have had an impact on economy too (they've all changed), as have the odd fuel leak, mother driving the car and of course slow punctured tyres.
The figures I keep are brim to brim, over the last 5 tanks I've varied from 46.5 to 48.7, same route, same driving style, same amount of tyre inflation. Even keeping things as consistent as possible and using the same pump at the same petrol station I've got quite a bit of variation.

jeremy · Post by **jeremy** » 28 May 2005, 12:53

Inevietably the fuel consumption is related to the driving you do and other things like the load in your car. To move the car takes energy which comes from the fuel - so the more energy required - the more fuel used.
The more efficient the setup - the more it will vary - diesels do not deliver any fuel when on the overrun and are particularily good on part throttle. Accelerate hard so that 'Extra fuel devices' start to work with the turbo and it will drink fuel, as Tom Sheppard found when he ran his BX TD at 100 with about 32 MPG if I recall correctly.
I'm sure that old economy techniques like build up speed downhill and let it drop a bit uphill, imaging you have an egg between your foot and the accelerator etc still work - and with effort I could probably get about 55 out of my BX TD and 60 or so from the ZX 1.9D. Best I've seen so far is about 52 for BX and 55 for ZX - both on long runs.
jeremy

adamskibx · Post by **adamskibx** » 28 May 2005, 22:16

Jeremy- When you say the deisel engine doesnt use any fuel on overun, do you mean wwhen the engine is being turned my cars momentum? This would mean its more ecconomical to leave in gear when slowing down etc than selecting neutral. I know new cars, both petrol and deisel are like this but i didnt realise old deisels worked in this way. Ill change my driving style if this is the case as im rather clutch heavy as a rule.

jeremy · Post by **jeremy** » 28 May 2005, 23:45

Yes - thats right - fuel is cut off on overrun. Actually most petrols with electronic fuel injection do the same thing over 1400 rpm which is why the engines slow sweetly when the throttle is released. Do the same thing with a carburrettor engine and it will probably splutter as the revs drop even if it doesn't backfire. It will certainly take longer for the revs to drop.
I shouldn't worry about changing your driving routine - they use very little at idle anyway but its not worth doing anything special.
Jeremy

FrenchLeave · Post by **FrenchLeave** » 29 May 2005, 22:23

I'm another who fills brim to brim and keeps a continuous record of fuel consumption. I have a PDA coupled to a GPS receiver in the car (courtesy of Aldi) for navigating and the results go straight into the PDA spreadsheet. I've put in a few simple formulae which do the calculations for me so all I have to put in is Fuel In (litres) and the current mileometer reading. Out comes Miles Done, Fuel In (gallons), Current Fuel Consumption and fuel consumption this year. Incidentally, the navigator is a super piece of kit that I've had for two years now. The female voice that gives instructions is nicknamed Clara and my wife hates her, she can't stand another woman telling me what to do. - "Please make a U-turn as soon as possible".

bikeboyz · Post by **bikeboyz** » 29 May 2005, 22:42

Handy fuel calculator, just completed the 380 mile round trip for FCS. It seems like the 504 managed 40.7mpg ! Not bad for a old girl nearly 28years old with very old diesel technology

Peter D · Post by **Peter D** » 05 Jun 2005, 17:10

Putting the car in nuetral provides the a much longer slow time put is in fact breaking the road traffic act. The only car that used to get away with it was a Saab that had a free wheel as part of the gearbox. Free wheeling over anything other than a short distance is a offence and can be deemed to be dangerous driving. As far as stoppong is concerned your car will slow down quicker in gear due to the engine resistance, particularly the deisel engines due to the high compression ration 20 to 1 and up. Regards Peter

TomH · Post by **TomH** » 07 Jun 2005, 22:01

I've only started keeping a record this year. My 306 1.6 has done an average of 36 mpg over about 4,000 miles of mostly urban driving with the odd longer run.
I fill up to the click, but am unsure about the reliability of this since my figures can be sometimes erratic.
My highest tank got me 39mpg while the lowest was 32!

TomH · Post by **TomH** » 07 Jun 2005, 22:03

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Peter D</i>

Putting the car in nuetral provides the a much longer slow time put is in fact breaking the road traffic act.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
exactly. Drive dangerously without full control to save a few pennies? [V]