calculated vs. computer MPG - Please post your results

Yesterday I gave it a shot at driving normally. I had to go to Delft, which is a 52 km roundtrip. The temperature was 30 C, we drove with the windows down in the city, with A/C on the highway. I had 220 kg of cargo in the form of 4 kids. I did not employ any special techniques, no coasting or P & G. The engine was cold when I left. I have the 15" wheels. EDIT: there was hardly any wind.

On the way there, I went right through the city centre of Leiden because there was a traffic jam on the highway. When I turned up the highway after 6 km, I was at 5.5 l/100 km. The highway has a 100 km/h speed limit, so I put the cruise control at 105 km/h. I arrived in Delft with 4.7 l/100 km on the MID. We returned one and a half hour later. Arriving back home, the MID showed 4.2 l/100 km. With the known deviation that should be around 4.4 l/100 km in real life.

4.4 l/100 km = 53 mpg. Easy, without really trying.

 

My last 2 fill ups have been as follows:

1. Car Calculated 52 MPG but my calculation was 49MPG
2. Car Calculated 54MPG but my calculation was 50MPG

I wonder where the discrepancy comes from.

 

fill up today:
miles 441.4
gal 8.923
MFD 54.8
miles to go 42
calculated 49.47

totals miles 3041
gal 58.59 computed MPG 51.9
average per MFD = 56.3

this just does not compute! still better than the '06 but we have another "guess gauge"????

 

fill up today:
miles 389.4
gal 8.993
MFD 46.8
miles to go 42
calculated 43.3

 

I have consistently seen the computer overestimate my MPG's by about 3 MPG's every fill up. It is really annoying and I wish they could have been more accurate. In 2000 miles of driving so far the trip computer says I have averaged 55 MPG but on paper it is 52 MPG.

 

Filled up today;

585.9 miles

60.1 MPG displayed

10.8 gallons filled

54.25 calculated

9% difference

Wayne

 

I'm still looking for more data points but from the public postings, this appears to be the trendline:


The data:

	Column 1	Column 2
0	indicated	actual
1	57.0	54.6
2	57.1	55.4
3	54.0	52.0
4	49.7	50.1
5	51.0	48.0
6	53.7	50.3
7	60.4	57.3
8	52.4	49.9
9	57.6	54.1
10	50.7	49.0
11	49.3	45.8
12	65.9	63.2
13	46.6	47.3
14	53.6	50.8
15	55.0	48.5
16	37.9	38.5
17	57.1	55.9
18	41.2	38.9
19	50.7	47.9
20	46.7	44.6
21	48.3	45.5
22	45.7	43.7
23	44.9	45.0
24	46.9	44.8
25	41.5	39.3
26	44.3	42.4
27	48.2	46.0
28	51.0	48.0
29	48.4	46.9
30	51.6	51.1
31	51.5	51.8
32	53.4	50.9
33	53.1	50.5
34	53.1	50.9
35	57.7	55.3
36	57.6	56.5
37	54.3	53.4
38	62.3	56.7
39	56.6	51.0
40	57.5	52.6
41	59.9	54.6
42	57.1	51.4
43	60.1	54.3
44	46.8	43.3
45	54.8	49.5
46	52.0	49.0
47	54.0	50.0

Bob Wilson

 

495miles 8.7gal. car computer said 59.9 actual 56.9

 

Good thought, but mine is 10% off consistently and even with a one day 300 mile trip. If the evaporation theory were true, I would see a bigger difference for tanks that take longer to use up. I think Toyota wanted us to think the car was doing better than it really is. I am getting pretty good at guessing my actual mileage before calculating it because mine is consistenly off the same percentage

 

I've added some more data points and taken the 'outliers', the bottom and top 10% out:


Removing the outliers tends to smooth the data but it can also raise questions about what is going on at the limits. This afternoon I added the upper data point, indicated: 76.4 MPG, actual: 64.0 MPG. The target speed was 35 mph on cruise control and the average speed measured 30 mph.

It would helpful to get more 'high mileage' numbers to see what is going on at slower speeds. As interesting as the indicated mileage error is, I'm fascinated to find out what is going on at slow speeds. Normally, slow speed mileage is normally driven by rolling drag and vehicle overhead.

Bob Wilson

 

Car bought late June. After 5 tanks, Calc and MID values for MPG seem to be smoothly converging with time (only exception was 4th tank which was filled "prematurely" with 89 mi DTE). Look like difference heading to zero?? Will keep checking this.

 

Hopefully I can help a bit. I just haven't been able to provide any data because I don't need to fillup often enough. I've had my car over 2 months and just got my second fillup today ... and didn't really want to trust the first tank, etc.

My normal commute is ~6.5 miles each way

miles gallons mpg(pump) mpg (displayed) error
519 9.2 56.4 59.1 + 4.5%
532 9.1 58.5 61.0 + 4.0%

As a comparison, this is about 8-10 mpg greater than I get in the same months in my 2001 and 6-8 mpg greater than in my 2004.

3PriusMike

 

Update, 24 August 2009; Filled up today, 432.4 miles driven;9.25 gal pumped; Trip A computed 50.0 mpg; calculator says 46.7459 mpg; Total milage on car Odo 2,736; Trip B reading 2,627.8 miles; Trip B says 43.6 mpg. All in all, not bad I think. I drove very carefully on this last tank and I don't see any way to improve. I think as long as I can stay 40 + mpg I will be happy.

 

We've added more points, 60 -> 80:


NOTE: I'm trying to use mileage numbers, indicated vs actual, from this thread and a couple of PMs. But it takes a few minutes to add records so it makes sense to 'batch update' the updates.

Bob Wilson

 

Stats update for my 2010:

Purchased 8-5-09
8-15-09 Calc'd 48.152 mpg, Comp 49.9 mpg (based on E10 gas)
8-23-09 Calc'd 56.02 mpg, Comp 58.2 mpg (based on non-E10 gas)

 

117 miles on the car when I got it.
First tank fill at 630 (reset the trip odo @ 192) - reported 58.4 on 437 miles, 53.2 on 629.5 miles (trip B avg when I got the car was at 44.1). Actual fill 9.449 gallons for a 54.3 MPG. The first driving was 52 miles r/t to work, 61.6 first "to", and then it degraded slightly from there.
The second fill was after a road trip with lots of highway (~70 mph) and hills at the end. 1098 ODO for a diff of 469 miles with a fill up of 9.465 for avg of 49.4 (52.3 reported). Avg 41 MPH.

My Jeep consistently over-reported as well, probably about the same percentage, although I don't have those #'s recorded in my Palm app.

Robbin

 

Following is my last tank result.

distance: 1043.5km(648.5.7miles)
gas pumped: 35.76L(9.45 gallons)
mileage indicated: 31.1km/L(73.2MPG)
mileage calculated: 29.2km/L(68.7MPG)
average speed: 67km/h(42MPH)

Ken@Japan

 

With 109 samples, manually collected from the PriusChat community, we now have enough data. I'll attach the spreadsheet in case anyone else wants to do more. First the calibration:

The upper, outliers are still sparse but they seem to be around the trendline. Not enough to 'bet the farm' but enough to not worry too much if extended.

The other interesting aspect is the double-humped distribution:

This closely agrees with the double-humped distribution from the small number of user mileage reports from the EPA database:


At this point, the ball is in Toyota's court. Absent some way to modify the software that calculates the indicated MPG, we just need to understand the problem and depending upon the requirements, there are these options:

• use pump MPG for accuracy and indicated for relative MPG performance
• use correction formula: actual = (.85 * indicated) + 5.3
• use 3d party MPG indicator: scangauge, supermid or others (mpguino)

Feel free to share this with other Prius groups. Thank you everyone for your contributions.

Bob Wilson

ps. I've attached my spreadsheet and folks are welcome to do with it what they want. Please acknowledge the contribution of PriusChat members who provided the data.

 

Well done, Bob. Thank you for gathering and analyzing this data. I'd like to see a few hundred more data points, of course, but you've clearly had enough to see a trend developing!

 

Bob, after 2482 miles tracked, here are my numbers and how they compare with your data:

Indicated MPG: 57.4
Actual (calculated by iPhone gas cubby): 55.1 (-4.006% relative to indicated)
Actual using your formula: 54.09 (-5.766% relative to indicated)

The gap between actual and indicated is actually quite significant in my opinion. The gap in my CR-V was always less than 3% and often around 1-2%.

Am I just misunderstanding it or does your data suggest that the higher the MPG the higher the gap between actual and indicated?