How's my maths?

40701085

I think my Accord is giving me fuel effciency readings that are waaaay over-optimistic.

Filled the tank last week, Set the trip computer & did a week of easy driving (almost constantly on N-primary roads), today the computer reading tells me 4.6L/100km.
But the trip computer also tells me I did 1010km. The car used 53.5L to do those kms (I know this cos I re-filled it the tank this evening & that's how much it took).

So my own calcs work out at 5.3L/100km, instead of the trip's 4.6L.

In old money a difference of 53mpg to 61mpg!

Who's right & who's wrong

sentient_6

Is it remapped?

wesf

go with the maths, i wouldn't trust a trip computer

Alanstrainor

My trip computer always tells me I'm getting 25+ MPG when there's no way I'm getting close to that. Always rely on your own maths over the on board computer.

Cicero

If you're accelerating in low gear, does the computer throw out mad figures like 40litres/100km which may indicate car is running rich ?

Indricotherium

The trip computer is not registering petrol used when the car is idling.

Technically this is correct as although it foes effect your pocket it does not effect the miles per gallon

dieselbug

1010 klm = 606 miles
53.5 L = 11.784gals

606 divides by 11.784 = 51.42mpg or 18.878 klm per litre

How did I do?

40701085

40701085 wrote: »

..............
But the trip computer also tells me I did 1010km. The car used 53.5L to do those kms.............

There's the info you need.

40701085

sentient_6 wrote: »

Is it remapped?

What does remapped mean? sorry

Cicero wrote: »

If you're accelerating in low gear, does the computer throw out mad figures like 40litres/100km which may indicate car is running rich ?

Don't think so, only get the mad figures at the begining of trip, i.e. as expected until the averages settle out over distance.

dieselbug wrote: »

1010 klm = 606 miles
53.5 L = 11.784gals

606 divides by 11.784 = 51.42mpg or 18.878 klm per litre

How did I do?

I'm ball-parking it at 53.27 -close enough to hardly matter!


Thanks folks, suppose t'was easier to believe the computer's "great" news, but as ye say the figures don't lie..

sentient_6

40701085 wrote: »

What does remapped mean? sorry

Thats a no then.

Remapping is changing the ecu parameters to give extra engine power. But on my yoke its made the trip computer fuel efficiency read better than actual. For instance i did 6.7 L/100km on my last tank but the computer is saying 5.9. Its consistently out by 0.7 - 0.9 every time. I've read elsewhere about this happening too. Before the remap it was reasonably accurate.

Jimbob 83

My trip computer says 23.2 MPG, it's a lying slut as im obviously getting atleast 30.

That's my story and im sticking to it anyway

mik_da_man

Jimbob 83 wrote: »

My trip computer says 23.2 MPG, it's a lying slut as im obviously getting atleast 30.

That's my story and im sticking to it anyway

LOL - U wish

Lucky to be over 20 tbh....

dieselbug

To get accurate figures all you really need to be concerned about is the klms clocked in total.
IE if you fill up, reset the trip, drive on for say 200 klms. Fill again and take litres to fill and klms covered.
The only thing that can throw you is if klms recorded is not accurate.
Hope I got this correct as Im just in from the pub.

Daveddub

Indricotherium wrote: »

The trip computer is not registering petrol used when the car is idling.

Technically this is correct as although it foes effect your pocket it does not effect the miles per gallon

This is the answer, why is everyone ignoring it?

baz.snn

The computer on my Caravelle is a small bit optimistic too. The only way to do it right is by doing the brim to brim method for each fill. Then you can average it out over a number of fills. That way then you get a realistic overall average figure. On a decent run my computer will be around 2mpg out. The real figure is usually 36-38 mpg

I use an app on my iphone called Road Trip.Its a very handy app to quickly view how much its costing you to run the car. You realise how much you are actually spending on fuel, quiet shocking really .:eek::eek:

Leonard Hofstadter

Indricotherium wrote: »

The trip computer is not registering petrol used when the car is idling.

Technically this is correct as although it foes effect your pocket it does not effect the miles per gallon

I've never heard of that happening before - on any car I've had the average fuel consumption will always decrease if the car is let idle.

It's a fact of life that trip computers are often very optimistic. That said, the trip computer in a 97-02 Corolla is so good it will sometimes under-estimate your mpg:D!

Eph1958

Indricotherium wrote: »

The trip computer is not registering petrol used when the car is idling.

Technically this is correct as although it foes effect your pocket it does not effect the miles per gallon

Trip computer on my Transporter switches to l/hr when idling for a few minutes and increases significantly with aircon on.

vandriver

dieselbug wrote: »

1010 klm = 606 miles
53.5 L = 11.784gals

606 divides by 11.784 = 51.42mpg or 18.878 klm per litre

How did I do?

You got it wrong.

1010 km=627.5 miles

mpg=52.3

Indricotherium

captainspeed wrote: »

I've never heard of that happening before - on any car I've had the average fuel consumption will always decrease if the car is let idle.

It's a fact of life that trip computers are often very optimistic. That said, the trip computer in a 97-02 Corolla is so good it will sometimes under-estimate your mpg:D!

Sure you're dividing by zero there. It makes no sense.

sebastianlieken

Just checking, but did you use the same reference for determining an "empty" fuel tank both times?

As in; was it the moment the red light came on that you filled it up and began your test, and then took the 1010km reading when the red light came on again?

Leonard Hofstadter

Indricotherium wrote: »

Sure you're dividing by zero there. It makes no sense.

Not so, the only time you'd be dividing by zero is when you start up the car straight after refilling it/resetting the trip computer. If you drive any distance at all, it is correct for a trip computer to work out the fuel used since the last time the trip computer was reset and the distance travelled. And that's what any trip computer I've come across does.

vandriver

sebastianlieken wrote: »

Just checking, but did you use the same reference for determining an "empty" fuel tank both times?

As in; was it the moment the red light came on that you filled it up and began your test, and then took the 1010km reading when the red light came on again?

The only accurate reference point is the car filled to the brim

sebastianlieken

vandriver wrote: »

The only accurate reference point is the car filled to the brim

but how do you know when it's empty?...

jlang

You don't. You know how much takes to get back to full. Your reference is when it's full, not empty. (as the others said)

Indricotherium

vandriver wrote: »

The only accurate reference point is the car filled to the brim

This is nonsense.

It is impossible to get accurate data to plug into an equation by 'brimming' a tank with an unknown amount of fuel in it.

The trip computer ignores fuel used on idle, and as such gives a figure that is accurate when discussing distance per unit of fuel, but may seem optimistic, when compared to distance per units of fuel put in car.

There naturally will be some difference between these two figures.

vandriver

Indricotherium wrote: »

This is nonsense.

It is impossible to get accurate data to plug into an equation by 'brimming' a tank with an unknown amount of fuel in it.

The trip computer ignores fuel used on idle, and as such gives a figure that is accurate when discussing distance per unit of fuel, but may seem optimistic, when compared to distance per units of fuel put in car.

There naturally will be some difference between these two figures.

How do you "know" that the fuel used on idle is ignored ?Certainly the instantaneous readout won't give you a figure for mpg because mathematically it can't.On my car the fuel used is certainly recorded,as my fuel consumption falls on an extended idle.And as for using the brim fill as an accurate reference point,it is exactly the same point in the tank each time (ie you can see the fuel),so I am lost as to why you think this is not accurate .

Indricotherium

vandriver wrote: »

How do you "know" that the fuel used on idle is ignored ?Certainly the instantaneous readout won't give you a figure for mpg because mathematically it can't.On my car the fuel used is certainly recorded,as my fuel consumption falls on an extended idle.And as for using the brim fill as an accurate reference point,it is exactly the same point in the tank each time (ie you can see the fuel),so I am lost as to why you think this is not accurate .

Because the amount in the tank is different every time, and impossible to measure.

vandriver

Ok,fill up the bath at home,up to say,the overflow(let's call this the brim).Now,submerge a pint glass in the water and withdraw a full pint of water and throw it down the sink.Repeat this 7 more times.I will concede that you have absolutely no idea of the volume of water in your bath(let's call it the tank just to humour me).
But,I've a shiny new euro to bet with you on how much bathwater you will need to replace to get the bath up to the brim mark again.

dgt

Thank fook I didn't wire the trip computer up, who knows what it'd do to the electrics :eek:

Or should I for the schneer and see what happens.... :pac::pac::pac:

Dr.Rieux

dgt wrote: »

Thank fook I didn't wire the trip computer up, who knows what it'd do to the electrics :eek:

Or should I for the schneer and see what happens.... :pac::pac::pac:

I can see the engine being out in the near future

Leonard Hofstadter

For the love of God, trip computers DO take into account when the car is at idle. It is total bullcrap to suggest that trip computers 'ignore' this. If you leave a car idle long enough, the average fuel consumption will decrease.

If you don't believe me, reset the trip computer, drive a mile or even two miles, come to a complete stop, and leave the car idle, and see what happens.

wonski

captainspeed wrote: »

For the love of God, trip computers DO take into account when the car is at idle. It is total bullcrap to suggest that trip computers 'ignore' this. If you leave a car idle long enough, the average fuel consumption will decrease.

If you don't believe me, reset the trip computer, drive a mile or even two miles, come to a complete stop, and leave the car idle, and see what happens.

Mine(Renault) does the same. It works it out from how much fuel is used for given miles, so - even if you are not moving - petrol is burnt, so the mpg goes down a bit. You won't notice it unless you reset it regularely - the more miles you clock, the less impact idle has on your (displayed) mpg.