calculated vs. computer MPG - Please post your results

2.1 gallons
that sounds like a lot
That is 17-18% of the whole tank capacity

 

Computer trip meter reports avg. 52mpg
I Calculated by myself for the same trip 48mpg

Stinks!

 

That is about 8% which is about ave. for all of us

I would also prefer a more exact reading

 

435.2 miles / 8.515 gals / 51.130 mpg / indicated 53.2 mpg
real world driving

alfon

 

I am getting about a 2mpg optimistic view from the Prius, very similar to my 2004 Prius.

 

For my first 5 fillups (2750 miles)

Reported = 60.4 miles per gallon
Actual = 57.4 miles per gallon

Roughly a 5% overestimate. I deal with it, despite my Quality background....and all the "anality" that comes with it.....

 

After 5 tanks on my 2010, actual is within a fraction of 5% under the gauge on every tank. I drive mine like I drive, never baby it so my actual mileage is close to 45mpg. 75MPH down the highway and I do my best to stay up with traffic. My EVO got 20/24 so I'm very happy with the purchase.

 

Last year, I collected this set of data from folks who volunteered their actual and indicated MPG:

	Column 1	Column 2
0	indicated	actual
1	37.9	38.5
2	41.2	38.9
3	41.5	39.3
4	43.6	41.3
5	44.3	42.4
6	44.6	43.2
7	44.9	45.0
8	44.9	45.0
9	45.7	43.7
10	45.7	43.4
11	46.5	45.2
12	46.6	47.3
13	46.7	44.6
14	46.8	43.3
15	46.9	44.8
16	46.9	44.8
17	47.7	46.8
18	47.8	44.7
19	48.1	45.7
20	48.2	46.0
21	48.2	48.0
22	48.2	45.2
23	48.3	45.5
24	48.4	46.9
25	48.4	46.9
26	49.2	47.1
27	49.3	45.8
28	49.3	45.8
29	49.5	46.2
30	49.6	46.4
31	49.7	50.1
32	49.7	46.6
33	49.9	48.2
34	50.0	46.7
35	50.4	47.2
36	50.4	47.2
37	50.7	49.0
38	50.7	47.9
39	50.7	49.0
40	50.7	47.9
41	51.0	48.0
42	51.0	48.0
43	51.0	49.4
44	51.4	48.7
45	51.5	51.8
46	51.5	48.8
47	51.6	51.1
48	51.6	51.1
49	51.7	49.4
50	52.0	49.0
51	52.0	48.8
52	52.1	51.9
53	52.3	49.4
54	52.4	49.9
55	52.4	49.8
56	52.5	51.8
57	52.5	53.2
58	52.6	51.4
59	52.7	48.8
60	52.7	51.2
61	53.0	50.4
62	53.1	50.5
63	53.1	50.9
64	53.1	49.8
65	53.4	50.9
66	53.6	50.8
67	53.7	50.3
68	53.8	52.6
69	54.0	52.0
70	54.0	50.0
71	54.1	51.2
72	54.3	53.4
73	54.8	49.5
74	54.9	52.3
75	55.0	48.5
76	55.0	51.0
77	55.0	52.8
78	55.1	50.8
79	55.1	51.1
80	55.5	51.2
81	55.5	50.3
82	55.5	52.6
83	55.7	53.3
84	56.0	52.3
85	56.1	51.5
86	56.3	56.0
87	56.4	52.9
88	56.5	48.6
89	56.6	51.0
90	56.7	54.2
91	56.7	54.0
92	56.9	56.7
93	57.0	54.6
94	57.1	55.4
95	57.1	55.9
96	57.1	51.4
97	57.1	55.4
98	57.1	55.7
99	57.3	55.2
100	57.4	55.6
101	57.4	54.8
102	57.5	52.6
103	57.6	54.1
104	57.6	56.5
105	57.6	54.1
106	57.7	55.3
107	57.7	55.7
108	57.8	55.9
109	58.0	55.5
110	58.2	56.0
111	58.4	53.2
112	58.4	54.3
113	58.5	55.8
114	58.8	55.3
115	59.0	55.9
116	59.0	55.2
117	59.1	56.4
118	59.2	55.2
119	59.3	56.2
120	59.9	54.6
121	59.9	56.9
122	59.9	56.7
123	60.1	54.3
124	60.1	58.0
125	60.4	57.3
126	60.4	57.0
127	60.7	57.3
128	61.0	58.5
129	61.3	58.4
130	62.0	59.4
131	62.3	56.7
132	63.6	53.8
133	63.6	59.7
134	65.9	63.2
135	68.1	62.4
136	73.2	68.7
137	76.4	64.0
138	80.5	71.4

PriusChat user data.

I did a series of analysis trying to identify the outliers and eventually came up with this chart:

I tried different approaches to find the outliers and this one matches my expectations for a linear offset:

1.0 - 0.9548 ~= 0.0452 = 4.52% error
​

The "0.9548" is the slope of the a linear trend line through the ~160 samples. As for the outliers, I tried trimming both ends but still had a Y-axis intercept problem, it didn't go through zero. But when I set a 10% threshold for outliers, the line came smack dab through the zero axis point.

Having the outliers all reside on one side of the user data suggests that their pump may have delivered less fuel than indicated on the receipt. So I would caution anyone who sees their indicated versus actual mileage to be off by 10% to 'note' that gas station and pump and try someone else. If the error goes lower, say closer to 5%, and returns to 10% or higher when going back to the original station, it is time to file a report with the state weights and measurements office. For good measure, send a copy to your state representatives and keep your records for an attorney. It may also be OK and just a 'rounding error' in the Prius indicated mileage.

Many years ago, Alabama had some pumps that if the dollar value hit on a certain, low digit number, the gas quantity would 'adjust' up. What this meant was you had to detect a pattern and it only had to thieve 1 out of 10 times.

Bob Wilson

 

Thanks Bob...good info! If anyone has an excel spreadsheet they use for their fuel logs that has formulas built into it that takes into account weighted figures for tanks based on miles driven...please pass it along as I'd like to incorporate this into my own fuel log (or just use yours)!

 

I tried to paste a table showing all my fill-up results since I bought the car last July...but unfortunately it threw away the formatting. Here are my lowest and highest readings. The lowest occurred during very cold weather after my usual short, stop-and-go commutes. The highest was after a 650-mile road trip with no traffic or weather issues. As you see, the variance is 5% for both (although I've had as low as 1% and as high as 8%).

Lowest (1/16/10)
Displayed MPG: 33.4
Calculated MPG: 31.77
Variance: 5%

Highest (5/29/10)
Displayed MPG: 54.60
Calculated MPG: 51.69
Variance: 5%

 

For me, over the course of 4 fill ups, the computer is averaging about 2.75 mpg optimistic.

Tank-wise I am averaging 53.5.

Was hoping for more as I was doing not too much less in my '03 in the warm weather.

 

Here are my numbers FWIW.

14 fill-ups:
average miles: 454.4
average gallons added: 9.04
average hand-calculated mpg: 50.3
average FCD mpg: 52.8

 

After ten fill-ups I am at 2.33 MID average over-predicting calculated. My average fuel mileage is 55.3 mpg and climbing now that summer has arrived (last 2 fill-ups averaged 59mpg). That is 4.2% over-prediction by the MID. I find that if I fill my tank with the same methodology each time (automatic pump click-off, then add by hand, one more click-off), I get pretty consistent MID readings, 4% high, plus or minus a few tenths.

 

Rick you are in the 85th-90th %tile for all GenIII drivers for mpg, and in the 99.9th %tile for all automotive owners. If you include oxcarts, rickshaws, horse buggies, bicycles, golf carts and other land vehicles, you are at the 99.99th %tile. So, relax and be proud.

 

2 fillups, MFD avg says 61.25 , Pump avg says 61.13 Pretty good agreement - 3/10 Date of manufacture.

 

I have 3 fillups. Total miles just slightly over 1100 and highway miles 850. Just made a long trip with A/C at 75 and cruise set around 70mph.
Car computer is showing an average of 50.1 mpg and calculated it is 48.53. I am very happy with these numbers.

 

computer 49.5 / calculated 47.6 mpg.

same pump, same pump speed. veh parked at same spot when filling.

had over 380 miles on tank.

last fill up was 2.5 mpg off in the positive after over 400 miles, same pump etc.


alfon

 

I have 19,000 miles on my 2010. The computer continuously overstates mileage by as much as 4mpg. It has varied as much as 5 and as little as 1. The higher the mileage the more spread is realized. In addition to this Toyota misscalculation, the speedometer is between 1 and 2 mph off compared to three different GPS's that I've used. I haven't taken the time to figure out what the real overstatement error is. I was getting 49-50 by the computer, 47-49 by the pen and paper method. Over the last four tanks, the mileage has dropped to 40-42 driving the way I normally do. If I turn everything off and really baby it I get 45 mpg. That's with my normal day to day driving. When I get on the highway (once every couple weeks to the inlaws) The mileage is the same as it was 48 mpg. Everyone is talking about the heat and AC dropping the mileage, but I live in Florida. It's been above 80 degrees for some time now. I have an apointment wed to get the charging system checked.

 

The weather in the northwest is getting warmer and the mpg's are
getting better.

here are the results.

note: same gas pump / shut off on first click at slow speed/ same vehicle location when filling up.

6/14/10 / 384.5 miles / indicated 49.5 / calculated / 47.632

6/20/10 / 289.5 / 51.8 / / 49.554

6/27/10 / 346.1 / 55.1 / / 53.131

Remember Oregon is 10% Ethanol ALL YEAR LONG

alfon

 

Average offset for 5 tanks of gas for me is the HSI reads 1.2 MPG over the pump calculated of 59.2....