2010 calculated vs. computer MPG - Please post your results

[Muhahahahaz]

Wow... so the MFD can be widely inaccurate?

In my case, I'm hoping that it's underestimating... I'm showing about 41 MPG on my current tank (100+ miles), but on the other hand, the fuel gage still has 9 out of 10 pips showing, which seems to indicate better mileage than that...

[alfon]

with well over 5,000 miiles on our 2010 Prius the MPG guage is consisitently off by 2-3 MPG,. I see other owners are reporting the same.

Alfon

[krisirk]

Originally Posted by Muhahahahaz

Wow... so the MFD can be widely inaccurate?

In my case, I'm hoping that it's underestimating...

It is not so much widely inaccurate, but more consistently ~5% overestimating the MPG.

Kris

[HTMLSpinnr]

I did another fill yesterday - 49.3mpg display (VERY hot, lots of A/C) and only 45.8 calculated. This was first click, no topping off. Graph is still consistently showing a ~3mpg difference between the two. (lifetime average 47.6 calculated, 50.4 screen to date)

Only reason I could think that the first tank wasn't so widely gapped is because I didn't do the fill (and it wasn't as hot).

The only other hypothesis would be whatever is being loss due to evaporation that isn't "counted" by injector pulses. I can't possibly quantify this, and the amount of volume with which evaporation can take place isn't that significant. It will be interesting, however, to see if this gap narrows significantly when things cool off in several months.

[yogadoc]

I filled up for the second time this week. I drove 636 miles on this tank, and the car took 10.060 gallons.

MFD: 65.9 MPG
Calculated: 63.2 MPG

The difference of 2.7 MPG is around what most people are reporting.

As an aside, this tank included a 300+ mile roundtrip to NY and back (mostly highway, with 2 passengers and luggage in trunk), which my V averaged 64.2 MPG on the MFD. Not bad.

I've been using Shell 87 so far, but it is ~$0.25 a gallon more than the other stations. Am I wasting money???

[billnchristy]

First fill-up: 46.6mpg displayed, 47.3 calculated.

[spiderman]

Finally my first tank of gas...

Miles: 468 @ 9.2 gallons
Displayed MPG: 53.6
Calculated MPG: 50.8

Right in line with most of the other posts.

Peter

[fuzzy1]

After five tanks and a bit over 1600 miles, my lifetime pump MPG is 54.6 mpg. Computing fuel consumed according to the MID mpg displays for each tank, it is overstating my mpg by 4.3%, or 2.3 mpg.

The last refill was sloped towards overfilling the tank, so I hope the error will fall with more time.

Does Prius compute fuel consumption from some actual volumetric measure, such as injector pressure and time? Or does it use indirect methods similar to ScanGauge? The later will tend to underestimate fuel use / overestimate MPG when alcohol or oxygenates are blended in.

[a priori]

I've now completed my first 10 fill-ups, and I've driven my new 2010 5,000 miles. It looks as though the car's calculated MPGs are consistently registering 5% higher than the fuel economy calculated by measuring each fill-up. This is the equivalent, for my mileage, of having the MID screen's FE at 2.6MPG higher than the number arrived at by calculating FE based on gasoline purchases.

Here is a chart showing the percentage by which the 2010's MID readout exceeds the calculated fuel economy. One line is for the percentage above for each individual fill-up, while the other line is the cumulative percentage by which the computer number exceeds the calculated one.

[bwilson4web]

Raw Data July 26 fill-up:

A-trip: 475 miles, 55.0 mpg, 32 mph average speed
Fill-up: 475 miles, 9.8 gallons, 48.5 MPG

Driving profile:

50 miles -> 52 MPG
120 miles -> 48 MPG (wife driving, not reset)
160 miles -> ~52-54 MPG (my return at legal speeds)
140 miles -> around town, ~60 MPG

Analysis:

I'm going to suggest that there is a flaw in the 'carry forward' that may treat the previous mileage, the last power-on cycle, MPG, as a 'starting point' and the current trip is added to that point for each power-on drive cycle. Rather than keeping a cumulative fuel burn like it currently keeps the mileage, it uses the previous MPG as a starting point everytime the car is started. This means an earlier, longer, terrible mileage segment gets 'eaten' and the displayed value will be heavily weighted by the most recent mileage.

This means the 'tank' display, whether Trip A or Trip B, can easily be off by variable amounts based upon the driving profile and not a constant error rate. In fact, it may seem somewhat random.

TESTING:

To test this hypothesis, we'll need to record each trip in a log while keeping the other trip meter set for the tank. Then we'll have to sum up the individual trip records with a spreadsheet and compare with the calculated value. For good measure, include one high speed segment of say 100 miles in the middle of a bunch of urban trips.

Bob Wilson