I can’t work it out.

[jaytip]

I’m trying to work out what percentage my trip computer is out by in my car.
I filled the tank, did 263 miles on a lovely trip up to the Cotswolds and then went back to the same petrol station and refilled it. My onboard computer said I averaged 64.2mpg and the cost per litre was 1.118
Based on the cars figures it should have come in at £20.80 (I think) to refill it, but it was actually £23.
So my question is, what percentage is my OBC overestimating by?
Cheers.

[Iain73]

I’m trying to work out what percentage my trip computer is out by in my car.
I filled the tank, did 263 miles on a lovely trip up to the Cotswolds and then went back to the same petrol station and refilled it. My onboard computer said I averaged 64.2mpg and the cost per litre was 1.118
Based on the cars figures it should have come in at £20.80 (I think) to refill it, but it was actually £23.
So my question is, what percentage is my OBC overestimating by?
Cheers.

If my GCSE maths still serves me well I will attempt a solution.

So to answer your question:

If your refill cost £23 you bought (£23/£1.118) 20.57 litres of petrol which is 4.52 gallons ( 1 litre = 0.22 gallon)

You did 263 miles / 4.52 gallons = 58.2 MPG.

The car says 64.2 MPG - actual 58.2MPG = 6.0MPG that wasn't achieved

6MPG/58.2MPG x 100/1 = 10.3% overestimation.

However, what you really want to know is what percentage of the displayed MPG is really delivered. In this case you need to divide 58.2/64.2 x 100/1 = 90.7%.

This now leads us to how do I look at my dash and work out the actual MPG = whatever it tells you x 0.907.

[prexelor]

∆ I think that looks like A level maths!

I have previously used an online MPG calculator. Much quicker.

[aa388]

I’m trying to work out what percentage my trip computer is out by in my car.
I filled the tank, did 263 miles on a lovely trip up to the Cotswolds and then went back to the same petrol station and refilled it. My onboard computer said I averaged 64.2mpg and the cost per litre was 1.118
Based on the cars figures it should have come in at £20.80 (I think) to refill it, but it was actually £23.
So my question is, what percentage is my OBC overestimating by?
Cheers.

Or the other question is. How far out is mileage shown on my car in relation to actual miles traveled?

[Omegamanic]

Could also be the petrol station - a couple I’ve used in the last 5 years regularly delivered more Litres at the pump than my bloody cars tank! I warned them both a few times, and things appeared to improve, with one of them having new pumps fitted not too long afterwards.

[Omegamanic]

Or the other question is. How far out is mileage shown on my car in relation to actual miles traveled?

This too. If a speedo has to over-read, does it also over-read the mileage?

[Rob777]

And they say we don’t spend enough money on education.... Pffft!

If my GCSE maths still serves me well I will attempt a solution.

So to answer your question:

If your refill cost £23 you bought (£23/£1.118) 20.57 litres of petrol which is 4.52 gallons ( 1 litre = 0.22 gallon)

You did 263 miles / 4.52 gallons = 58.2 MPG.

The car says 64.2 MPG - actual 58.2MPG = 6.0MPG that wasn't achieved

6MPG/58.2MPG x 100/1 = 10.3% overestimation.

However, what you really want to know is what percentage of the displayed MPG is really delivered. In this case you need to divide 58.2/64.2 x 100/1 = 90.7%.

This now leads us to how do I look at my dash and work out the actual MPG = whatever it tells you x 0.907.

[Iain73]

This too. If a speedo has to over-read, does it also over-read the mileage?

Well, if this was the case and the trip computer is correct the car would have to under-read the mileage.

If 64.2mpg was achieved, from 4.52 gallons the car would have travelled 64.2 x 4.52 = 290 miles. If the speedo displayed 263 miles it would mean it is under reading by 9.3%. I think this is much less likely as this means if you buy a car with indicated 50,000 miles on the clock it would actually have travelled 55,133 and trading standards would have been wise to it by now!

We have ignored for the sake of this problem the effect of temperature on the density of fuel

[CardShark]

Surely it's simply a case of 20.80 divided by 23 = 90.4%? Then multiply whatever the trip computer says in the future by 0.904 to give an accurate mpg, presuming that pump and odometer readings are accurate. No need to work out anything else.

[ernestrome]

What if it is not a percentage out? It might be a fixed amount which will give a different result.

It could also be a variable amount out of if the full capacity varies slightly.

[JonRA]

Surely it's simply a case of 20.80 divided by 23 = 90.4%? Then multiply whatever the trip computer says in the future by 0.904 to give an accurate mpg, presuming that pump and odometer readings are accurate. No need to work out anything else.

Yes, keep the maths simple!

[Ares]

Isn’t it also the case that it’s not a straight equation? Standing with a running engine, accelerating, going up and down inclines, air conditioning- all will impact on fuel consumption.

[Iain73]

Surely it's simply a case of 20.80 divided by 23 = 90.4%? Then multiply whatever the trip computer says in the future by 0.904 to give an accurate mpg, presuming that pump and odometer readings are accurate. No need to work out anything else.

Nope, because this assumes the price of the petrol was the same when the tank was filled the first time. We only know the price of the petrol at the second fill.

[MartynJC (UK)]

Isn’t it also the case that it’s not a straight equation? Standing with a running engine, accelerating, going up and down inclines, air conditioning- all will impact on fuel consumption.

This. Imagine you had been at a stand still for 1/2h in traffic (an example) you could be burning fuel but doing 0mph or 0mpg but used say 1/4g fuel (making these numbers up).

The onboard figures are only estimates based on averages - so because of variables will not be exact. A 10% variance seems reasonable.

It’s like my car says my spot miles/gallon goes to 99.99 when going down hill with no accelerator.

[Boss13]

Agree with Iain's initial maths...By the way which car do you have? That is very economical!

[Kingstepper]

This. Imagine you had been at a stand still for 1/2h in traffic (an example) you could be burning fuel but doing 0mph or 0mpg but used say 1/4g fuel (making these numbers up).

The onboard figures are only estimates based on averages - so because of variables will not be exact. A 10% variance seems reasonable.

It’s like my car says my spot miles/gallon goes to 99.99 when going down hill with no accelerator.

Shouldn’t affect OP’s problem. Time at standstill etc. will be taken into account by his trip computer and hence overall mpg figure.

[jaytip]

Thanks for the replies folks. Some interesting maths and points.
I can’t really grumble at the overall mpg though considering a lot of the driving was around the narrow Cotswold lanes so a lot of stop/starting and hills etc.

[jaytip]

Agree with Iain's initial maths...By the way which car do you have? That is very economical!

It’s a 2019 Merc E220. It is MILES better (pun intended) on fuel than my other 2016 E220 that I have now passed on to the wife.

[ernestrome]

What do you think the variance is of the 'full' capacity? Could it be as much as +- 2l?

[Iain73]

It’s a 2019 Merc E220. It is MILES better (pun intended) on fuel than my other 2016 E220 that I have now passed on to the wife.

It certainly knocks the (indicated!) 24.9 average MPG of my 2018 GLE 350d into a cocked hat.

[broxie]

Could also be the petrol station - a couple I’ve used in the last 5 years regularly delivered more Litres at the pump than my bloody cars tank! I warned them both a few times, and things appeared to improve, with one of them having new pumps fitted not too long afterwards.

Fuel is actually pumped/sold in kilograms (hidden from the purchaser) as you're purchasing calorific value. If it's hot, the fuel is less dense so you get more litres than you do if it's cold. Many pumps now have a tiny little sticker explaining the standard atmosphere where a litre bought actually equals a litre going into your tank.

Commercial aircraft always load up fuel by the kg, the litre quantity varies depending on temperature but 1 kg of fuel will always provide the same amount of energy when burnt regardless of temperature, it's just it's specific gravity (sg) changes.

A call to your local council "weights & measures" department (if they still exist) will have them check the garage pump calibration.

[jaytip]

It certainly knocks the (indicated!) 24.9 average MPG of my 2018 GLE 350d into a cocked hat.

Ouch !