Trip computer inaccuracy

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Sun 25 Apr 2021 06:43
Trip computer inaccuracy - Pedro Portela
Ever since I purchased this used SUV I have been extremely happy with it but for a couple of things. One is that this model doesn’t have room for a spare tyre (which apparently more than 10 years later since the car was launched is actually a significant market trend; I can live with that). The other, is that I seem to be consistently getting unrealistic readings from the trip computer.

When I say unrealistic, I’m not talking about the discrepancy one would expect of 2 or 3 or even 5 MPG. We are talking about around 20mpg. The car consistently displays for motorway majority trips around 55mpg of fuel consumption. At the time, I thought it too good to be true; after all, I had come from a 2L Diesel Toyota Auris and that one did about 45mpg.

The reality is that the RAV4 does about 30-35mpg. And this is what I would expect from a small diesel SUV, so I’m not complaining about the performance/consumption but the inaccuracy of the trip computer.

Has anyone here experienced something similar? And crucially knows if this is easy or affordable fix or it’s something that’s not worth doing (considering that’s not a real problem).
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Sun 25 Apr 2021 09:47
Trip computer inaccuracy - Andrew-T

I hope your calculations are correct (you imply that you have something to compare the readout with). I would not be satisfied with a diesel vehicle only doing 30-35 mpg. My Pug diesel will easily beat 60mpg on a motorway trip, tho admittedly I don't push along.

On the face of it your car's computer looks more credible than your calculations. I don't see how the miles you calculate from can be wrong, as the readout must use the same data. I think you need to check the 'reality' you mention. For example 1 gallon = 4.55 litres, or 11m/l = 50mpg.

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Sun 25 Apr 2021 10:17
Trip computer inaccuracy - Pedro Portela
I expect a small car to do 50 or 60mpg, but not an SUV. But I might be wrong on that as this is the first Diesel SUV I’ve owned.
Here’s now I calculate:
1. Fill up the tank (60L)
2. I ride the until it reaches the reserve
3. I fill up the tank again and see how many miles I did.
4. Convert miles to Km and calculate a rule of 3 to see how many Kilometres I would’ve driven with 100km
5. I then convert the L/100km using Google’s converter to imperial MPG.
If you have a different/better method would love to hear it. And more, I’d like to be wrong about the car!

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Sun 25 Apr 2021 11:19
Trip computer inaccuracy - FP

"Convert miles to Km and calculate a rule of 3 to see how many Kilometres I would’ve driven with 100km."

I've always struggled with maths, but I don't understand the statement above at all.

FWIW, this is how I calculate fuel consumption:

- Go to fuel pump. Zero the trip odometer.

- Fill the tank to the brim, carefully coaxing in as much fuel as possible until it's about to overflow.

- Drive the car until the tank is pretty low. Go to fuel pump.

- Note the miles covered, record, zero the trip odometer for next time.

- Fill the tank to the brim as before. Note the volume of fuel used, which will be in litres. (It's probably going to be on the receipt anyway.)

- Perform this calculation: multiply the litres of fuel used by .22 to give gallons. Divide the miles covered by the amount of fuel in gallons. The result is your consumption in MPG.

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Sun 25 Apr 2021 15:43
Trip computer inaccuracy - Bolt

Fill the tank to the brim, carefully coaxing in as much fuel as possible until it's about to overflow.

Should stop on first click not to brim, could cause unwanted problems....

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Sun 25 Apr 2021 16:05
Trip computer inaccuracy - FP

We've had a discussion on this in another thread. I've had no problem brimming any of my cars.

Though brimming is subject to inaccuracy if the ground isn't level, relying on the pump cut-off is even more unpredictable - pumps vary widely in when they stop.

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Sun 25 Apr 2021 13:55
Trip computer inaccuracy - Andrew-T
3. I fill up the tank again and see how many miles I did.
4. Convert miles to Km and calculate a rule of 3 to see how many Kilometres I would’ve driven with 100km
5. I then convert the L/100km using Google’s converter to imperial MPG. If you have a different/better method would love to hear it. And more, I’d like to be wrong about the car!

What a roundabout method. Can't see any point in converting miles to km and then back again - why introduce extra chances of error ?

Find miles; divide by litres; multiply answer by 4.55. What could be simpler ?

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Sun 25 Apr 2021 16:16
Trip computer inaccuracy - skidpan

Find miles; divide by litres; multiply answer by 4.55. What could be simpler ?

Looking at the above I am stunned to the complexity people are resorting to for such a simple calculation.

The one above is perfect, its how I have done it for years. Prior to that we bought in gallons, slightly easier then.

As to the accuracy of dash displays I have found they vary between cars. The most inacurate I have owned was a BMW which was about 10% out but a Ki we owned actually read about 0.5% low.

But the trip meter or odometer on cars is never totally accurate. Without knowing how fat it is out you are introducing am error into the calculations. Examples are a Golf which read about 5% high and a Kia that read about 5% low. Fords have always seemed to be very close. But don't forget that the accuracy changes slightly as the tyres wear.

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Sun 25 Apr 2021 17:55
Trip computer inaccuracy - Andrew-T

<< But the trip meter or odometer on cars is never totally accurate. Without knowing how far it is out you are introducing am error into the calculations. Examples are a Golf which read about 5% high and a Kia that read about 5% low. Fords have always seemed to be very close. But don't forget that the accuracy changes slightly as the tyres wear. >>

Those are second-order corrections, unless your odometer is seriously inaccurate, and any tyre correction is probably third-order, even if you don't bother to keep the tyre pressure up.

Even if you calibrate the odo using a sat-nav there will be a residual error. I try to keep my sums simple by adding a round number of litres to the tank, instead of brimming, which I avoid if possible.

For total accuracy, don't forget to compensate for any distance travelled in reverse, which will use fuel but decrease distance (at least on a mechanical odo (how does a digital odo handle reversing? it should be able to advance the distance ?) )

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Mon 26 Apr 2021 09:23
Trip computer inaccuracy - Andrew-T

... (how does a digital odo handle reversing? it should be able to advance the distance ?) )

I presume that a digital system takes signals from sensors on the hub, which won't distinguish forwards from backwards ?

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Mon 26 Apr 2021 11:07
Trip computer inaccuracy - Bolt

... (how does a digital odo handle reversing? it should be able to advance the distance ?) )

I presume that a digital system takes signals from sensors on the hub, which won't distinguish forwards from backwards ?

reads forwards, reverse is the same, afaia. not certain of EVs without gearbox, please correct if wrong ?

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Sun 25 Apr 2021 23:23
Trip computer inaccuracy - Andrew-T

<< Find miles; divide by litres; multiply answer by 4.55. What could be simpler ? >>

Better still, when car needs fuel, put in 45.5 litres (most cars can take that much) and record mileage. When next need fuel, calculate distance and divide by 10, as you have used 10 gallons.

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Mon 26 Apr 2021 11:21
Trip computer inaccuracy - John F

Better still, after initial brimming, drive for at least 1000 miles with multiple refills, before the final brimming, to minimise the inevitable inaccuracy of just one 'brim' difference.

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Mon 26 Apr 2021 13:48
Trip computer inaccuracy - galileo

<< Find miles; divide by litres; multiply answer by 4.55. What could be simpler ? >>

Better still, when car needs fuel, put in 45.5 litres (most cars can take that much) and record mileage. When next need fuel, calculate distance and divide by 10, as you have used 10 gallons.

How do you define "car needs fuel" . Position of gauge needle? When the low fuel level comes on?

Filling to pump shut off or brim, recording mileage and re-filling to shut off (ideally at the same pump) and then doing the [miles/litres] x 4.55 will be a more accurate method, surely?

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Mon 26 Apr 2021 16:21
Trip computer inaccuracy - Bolt

Find miles; divide by litres; multiply answer by 4.55. What could be simpler ?

Too simple for some it seems;) lol.

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Mon 26 Apr 2021 16:45
Trip computer inaccuracy - Theophilus

Find miles; divide by litres; multiply answer by 4.55. What could be simpler ?

Too simple for some it seems;) lol.

And if that's too complicated (!), the web has a converter www.mpg-calculator.co.uk/

... all the work done for you.

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Mon 26 Apr 2021 18:59
Trip computer inaccuracy - Andrew-T

<< How do you define "car needs fuel" . Position of gauge needle? When the low fuel level comes on? >>

Whichever you find more reliable - cars will differ. I have had cars with a gauge needle that travelled quite quickly past the quarter-tank mark. Others with a warning lamp that either began to flicker, or became steady instead of intermittent.

I just don't like driving with a brimmed tank, when alternative methods of calculating are available.

Edited by Andrew-T on 26/04/2021 at 19:00

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Wed 28 Apr 2021 09:27
Trip computer inaccuracy - Pedro Portela
Good morning,
So, followed the advice here and went for a simpler method.
I used up half the tank and filled that up.
Having ridden 150 miles using roughly 4 gallons, that gives me 37.5 MPG, which is alright for an SUV driving 60/40 motorway/urban. The on board computer gives me 63mpg. There’s clearly something wrong with the meter.
Thank you all for your help.
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Wed 28 Apr 2021 09:38
Trip computer inaccuracy - Andrew-T
Having ridden 150 miles using roughly 4 gallons, that gives me 37.5 MPG, which is alright for an SUV driving 60/40 motorway/urban. The on board computer gives me 63mpg. .

It's nothing to do with miles vs. km, is it ? Your discrepancy is suspiciously close to 1.6 ?

37.5 mpg == about 61 kpg .

As for 'something wrong with the meter' : presumably the distance is recording correctly, so you must assume the fuel metering is at fault. That is monitored by the injection system, so if the engine runs correctly that should be OK. Needs investigation.

Edited by Andrew-T on 28/04/2021 at 09:42

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