fuel consumption calculation march issue pc page47

[haydn_374343028]

formula is wrong.

it should read divide miles by gallons

also try divide litres by 4.54 gives gallons then divide miles by gallons.

most of us dont use gallons per mile if you do!!! change your car!!

 

[nrdonnelly]

Haydn,

Thanks for the note. I'll take a look and get a correction together.

Sorry for any panic it's caused at the pumps...

Regards

Nigel Donnelly

Editor

Practical Caravan

 

[WatsonJohnG]

I always wait for the low level warning light to come on and then the tank takes 50 litres which is 11 gallons

Allowing for 4.5 liters per gallon its easy to get gallons used

I always do a mental arithmetic miles/gallons but the mpg NEVER is as good as the car computer say it is !!!

 

[RogerL]

John - the car mpg display is usually optimistic - my last Vauxhall was 3.8% high and my current Subaru is 6% high! - that's recording pump quantity against car odometer over their full life, 98,000 miles in the case of the Astra!

 

[ProfJohnL]

I am always suspicious of the cars computed mpg figures, because it is based on two measurements that in them selves are suspect.

Most car speedometers are slightly optimistic and read faster than reality, and as most odometers are driven from the speedo system, they two are going to be optimistic as well.

I am not familiar with details of the fuel measurement system used in the car, so without any information as to its accuracy, I will always consider it to have some error.

It is of course possible that the two errors could cancel each other but it is more likely they will compound each other.

 

[RogerL]

The error introduced from the odometer source should be the same however fuel consumption is calculated - a typical 3% error means that a car only covers 9,700 miles for each 10,000 recorded.

Trip computers work by sampling the injection timing signal - sampling will never be 100% accurate - the actual injector opening time, compared to the signal, will vary due to gumming etc.

Trip computer mpg is a useful guide to consumption, no more!

 

[rob_jax]

Hi guys,

The usual way for a modern car to measure speed and distance is to use a Hall-effect sensor (magnetic switch) on the drive shaft - this counts rotations and the car computer converts this into distance and speed.

The wonders of modern electronics mean that the speed reading can be "adjusted" by the manufacturer to over-read (my E320 displays 62 at a true 60 mph) - so that you could never blame the manufacturer for a speeding ticket.

But the distance reading could and should be accurate - well at least as accurate as tyre wear and different tyre brands allow - and the distance part of the measurement will be consistent between a manual fuel log and car computer.

The variation lies at the door of the fuel flow sensor - and you'd be lucky if these show an accuracy better than plus/minus 5% (mainly because they are un-calibrated) - but they are very reproducible.

If the car display says 45.3 mpg - then you should expect the same specific consumption figure driving the same route under identical conditions. Commuting past Brugge on the "empty" section of the motorway to Gent, I see the same numbers most journeys (car limiter set to 120 kph) - but what the actual consumption is, I have no real idea.

Robert

 

[haydn_374343028]

Haydn,

Thanks for the note. I'll take a look and get a correction together.

Sorry for any panic it's caused at the pumps...

Regards

Nigel Donnelly

Editor

Practical Caravan

thanks nigel.

only tried to help! didnt expect such a plethora of tech advice and response from everybody.lots of clever people out there.