On Board Mpg Overreading

[Glyyde]

I know that on board MPG always overread by around 4 MPG but today mine over-read by 7.5 MPG. Was 63.9 on board and 56.4 on Fuelly. Is this because of the hot weather?

[Bibsy]

Are you sure you didn't have fuelly set to American gallons?

[johalareewi]

Might be because you got more into your tank this time so the fuelly mpg was a little lower than usual.

Fuelly will average out after a few readings.

[aCactus]

It's the hot weather, this causes the road to expand so the miles are longer :)

[DMGbus]

I have documented the difference between displayed mpg and true (measured) mpg.

Over a three month peiod in Spring/Summer 2005 I found that the displayed mpg averaged 62.2 mpg whilst the measured / computed mpg was 59.9 mpg. This was for a 2nd gen 2004-new Prius.

Lifetime mpg for a 2009-new (3rd gen) Prius from September 2009 to February 2011 was a displayed reading of 55.5 mpg whilst the true measured mpg was 52.6 mpg.

Established pattern on some Prius forums is an overstated mpg on the display and no known explanation.

[Opifex]

My actual mpg is always about 3mpg lower than the displayed figure. My guess is it's a design feature to encourage you to boast a bit more about how good these Prii are.

[Sooty]

"Established pattern on some Prius forums is an overstated mpg on the display and no known explanation."

My guess, FWIW, would be thus:

The Speedo reading is designed to be approx. 5% high, as dictated by European law.

The Odometer should be accurate, with no discernible under or over reading.

The on board computer will probably be reading the fuel flow against the indicated speed divided by time.

Therefore the computer will guesstimate the MPG from the speedo reading divided by fuel used ( passing through a meter of some sort) and the time taken to use the fuel.

I hope I have not confused too many people s I am starting to get too deep for myself. LOL.

[wass]

Our Prius over- reads by about 3 mpg and our CT200 over-reads by 3 mpg. This doesnt seem to be anything unusual since my previous Volvo S60 over- read by about 5%, my previous Honda CRV over-read by about 5%, my previous Audi A4 Avant over read by about 5% and my previous 1984 Jaguar sovereign XJ6 over-read by about 5%. To summarise, every car I have ever owned which has been fitted with a trip computer has over-read by about 5%. In some ways, it was easier when the car didnt have a trip computer, I would keep a note of fuel and mileage in a little book and would work out fuel consumption tankfull by tankfull. My thirstiest car without a trip computer was easily a 1975 Jaguar XJS 5.3 litre petrol automatic which had a best fuel consumption of just 9 mpg. It is hardly a surprise that jaguar didnt fit fuel computers to this model.... thankfully! It had a 20 gallon petrol tank and I swear that I could see the fuel gauge moving downwards at red traffic lights.

[Glyyde]

I do not have problem with over reading but rather was wondering about the inconsistency of it. I normally overt read by 3-4 mpg but its 7mpg over read this time that makes me wonder. ....

[Opifex]

Sorry, as often happens we've drifted off the topic again.

The difference between British and US gallons is 20% so is too large to explain this case.

Johal may be on to the right track but it's difficult to tell without knowing how many litres were put in. Differences between when pumps cut off will have a bigger effect if the total was only 15 litres than if it was 30 litres.

[EDIT 05/05/13 05:54] the reference to US gallons is because there was a suggestion earlier in the thread that suggested Fuelly may be set to the wrong units.

[Glyyde]

I am actually living in the UK.