Accuracy Of M.p.g. Displayed

[Sagitar]

I have seen a number of comments about the way in which the in-car display over-estimates the m.p.g. achieved.

In all the time that I drove the Prius T-Spirit, the accuracy varied somewhat, but always over-stated the m.p.g. achieved when compared with a manual calculation.

Today, I topped up the PiP and for the first time saw a significant under-estimate of the m.p.g. achieved.

The figure displayed for the period since the last top-up was 129 m.p.g. while the manually calculated figure was 134 m.p.g.

The figures themselves don't mean all that much in isolation since they depend up the amount of electricity used, but I thought it interesting that the computer could generate an under-statement.

I guess it must be more difficult to deal with the integration of small inaccuracies in the PiP.

[Ten Ninety]

I've seen a couple of under-estimates in my Gen3 and the wife's Auris HSD. They have been rare, though, and never as high as your 5mpg.

I suspect that at least some of the fluctuation in 'accuracy' of the on-board display might actually be down to variance in the 'real' calculations. Obviously, these figures are dependent on the petrol pump being used and the extent to which you can actually 'brim' the tank. This probably doesn't make much noticeable difference in a 30mpg car but at 130mpg the impact will be greater. 1 litre less in the tank thanks to an over-zealous pump shut-off will probably net you a few mpg more on your 'real' calculation, whereas in a 30mpg car it would be a mere fraction.

Whatever the cause, I find the fluctuation more frustrating than the over-reading as it makes it harder to really analyse the impact of differences in driving styles, journeys etc. Last week I filled up with the on-board display showing 76.1mpg and the Fuelly calculation was 75.6mpg. That's one of the closest readings I've had. The week before, the on-board display was showing 75.0mpg but the Fuelly calculation was only 69.7mpg which was the 'worst' inaccuracy I have seen since owning the car.

Both weeks were exactly the same journeys, driven in (as much as it is possible) exactly the same way. However, I had to use a different pump from my usual one when I filled up last week so it will be interesting to see if this coming week shows a wider inaccuracy if I go back to my regular corner of the forecourt.

[barrycoll]

Interestingly, (but maybe just slightly off topic) I have just had 2 trips to Heathrow airport, and both in a Prius, provided by Greentomato cars

The first driver drove like a 19 year old, with full Power always on the HSI, or hard on the brakes and so full re charge...he didn't know or care about his mpg, and it was only my suggestion that he changed his Display to show overall mpg, which was 54.9, which must be over the life of the vehicle, as there would never have been a reset....the car had done 41,000 miles in his hands

The return trip was with a different Prius/driver combo, and he had his Eco mode button set (maybe by acccident?), but also didn't know about the Display options, and also changed it to show overall mpg....he drove gently, cruised, and accelerated normally, but had no interest in mpg either, but his overall display in a similar 40,000 miles was 58.9 mpg

deducting 10% for misreading, and the difference is really marginal considering the different drive characteristic, and the fact that the driver has to pay for the fuel does not seem to be a driving force (as it were).....both cars were one year old T3s

both drivers were more concerned about being ripped off by their employer than the possibility of increasing their personal mpg

[Nicolai]

Normally, my OBC is around 3 mpgs high. At the last tank however, real world calculation was 6 mpg lower. Don't know what happened there.

[Westender]

My Gen3 T Spirit over estimates between 4-5 mpg.

[Grumpy Cabbie]

Some have said using different sized tyres makes the trip computer become accurate. Was it designed for a certain tyre size which was then changed when the car was released?

I suppose it makes sense?

[PeteB]

I've religiously recorded every fill-up for all the Prius I've driven over the last 12 years (over ¼ million miles) [being the sad individual that I am] and found the calculated mpg on average to be 3-4 mpg better than my calculation by dividing the volume pumped in into the miles driven. That said, there are so many opportunities for error, even the odometer can be up to 10% fast (which makes the difference even worse!).

By far the biggest variances were at pumps where the forecourt was tilting. A particular Gulf station I used to use near where the M25 crosses the M1 has a severely sloping forecourt such that the filler corner was highest. If I filled (gen 1 and 2 Prius) at one particular pump near the most steeply sloping part, I could get nearly 2 more gallons in than normal - and do some 200 miles before the first bar disappeared from the gauge. The Gen 3 seems to hold about 2 gallons less than the Gen 1/2, and this effect seems less pronounced.