calculated vs. computer MPG - Please post your results

I have been following your discussion of the variance between the actual calculated mpg and the computer mpg. I have observed that in 3 tanks the actual and the computer are the same. I reset trip A when I got home from the dealer and filled tank when I first purchased the car. Trip A now has 1569 miles and total fuel used 28.5 gal. for 55mpg. The computer says 55.1mpg. Each tank has registered higher on the car computer. Each individual tank is closer to 57.0-57.5 on trip B. I filled today 502 miles and 9.1 gals or55.16 and the computer was 57.1. So, my long term milage is accurate, but the indvidual tanks are always reading high on the car computer.

 

My bother is driving my Prius a lot more than I am so he did some tests with the A/C on. He said the auto A/C have negligible impact in FE if kept 74 degrees and above. He felt anything less than that was having some negative effect. He also concluded that outside the auto mode the A/C does have a significant impact in the FE. His conclusions were made driving in the ECO mode all the time with average high outside temperatures in the low 80s (it has been a very mild summer in IN!)… So I’m sure people in the south would have different figures.

In September, when my brother will be back to Brazil, and I will start my weekly commute to WI, I will evaluate several conditions and their impact in FE. In my drive to WI, I will have the car loaded (perhaps 600 lbs) but in my way back it's going to be much lighter, so I will test the impact of cargo weight on FE. Also, September will still have good climate conditions for traveling but it will progressively get colder, so I will be able to test the impact of could temperatures over time. I'm also planning to do trips in different speeds to test the impact of speed. It will be a fun commute, as you can see...

 

Now if you were coming from:

• Eastern Tennessee, Kentucky, Alabama, or Western Georgia, North Carolina or West Virigina - I'd have no doubts about the cargo. <GRINS>

Just make sure that cargo cover is workin'. <GRINS>

Bob Wilson

 

Anyone know of a good Blackberry app for tracking fuel economy/usage? I know many of us are iPhone junkies but personally, I'm stuck with a Blackberry.

 

I will be going from 273 to 289 meters of altitude from Indy to Madison, so pretty darn flat way, but still 600 lbs of cargo in the Prius will likely have an impact. We'll see.

 

Based on my experience, 600lbs of cargo will have a very large performance hit. I didn't zero the trip, but I was at 54mpg for the first 200mi of a trip. Then I had three more full sized people join me (about 550lbs) for the next 200 mi, and the moving average dropped to 45mpg after 200 mi. Now I am driving alone again, and am climbing back up slowly.

 

Was that a mountainous or flat road? I may not have that much impact because my trips will be very flat most of the time.

 

Toyota is looking at the reported inaccuracies of the FCD on the 2010 Prius. Look at this thread for more info.

 

Pretty flat, at pretty optimal speeds (55ish).

 

Interesting! Thanks!

 

I have some preliminary data but it does not include the high-power settings, yet:

	Column 1	Column 2	Column 3	Column 4	Column 5	Column 6	Column 7	Column 8	Column 9	Column 10
0	miles	MPG	speed	A gas	miles	MPG	speed	B gas	B sum	err
1	3.1	57.9	26	0.054	3.2	54.6	22	0.059	0.059	9.5%
2	14.5	65.8	17	0.220	11.4	70.3	16	0.162	0.221	0.2%
3	23.6	64.4	21	0.366	9.1	62.3	36	0.146	0.367	0.1%
4	25.4	66.4	21	0.383	1.7	99.9	16	0.017	0.384	0.3%
5	37.9	60.3	25	0.629	12.5	51.4	38	0.243	0.627	-0.2%
6	42.5	60.6	20	0.701	4.5	63.8	8	0.071	0.698	-0.5%
7	53.3	61.3	21	0.869	9.9	64.7	29	0.153	0.851	-2.2%
8	54.8	59.5	21	0.921	2.5	36.6	17	0.068	0.919	-0.2%
9	64.3	55.5	19	1.159	9.4	40	11	0.235	1.154	-0.4%
10	67.7	56	18	1.209	3.3	67.4	13	0.049	1.203	-0.5%

I can replicate my wife's trip to Nashville by re-running my hill climb runs that will put the car in maximum engine power modes. There are two ways to measure fuel consumption: (1) rpm and injector timing or (2) mass flow and fuel ratio. If mass flow rate is used, it might be fooled by high power settings that cause significant amounts of cooled exhaust to be injected.

Bob Wilson

 

At 254 miles:
MFD: 37.9 mpg
Calculated: 38.5 mpg

This is a big disappointment, and I have no explanation, except several short trips. Our 2006 has always done better than this.

 

How can you be so disappointed? You know that the numbers are going to increase over time, anyway.

Do you recall the first numbers for your 2006? Even if they weren't as low as 38MPG, they were much lower than what you get now. I know it was like that with my 2007.

For my new car? I don't even have the first 311 miles as part of any calculation. For two reasons: 1) I don't know how "full" was "full" for the dealer, nor how much driving vs. sitting with the ICE running actually happened; and 2) Both the dealer and I were busy trying to figure out how things worked, how quickly it really could accelerate, how well the A/C worked, etc. None of those actions mimic real world driving.

Why not just chalk up the first 254 miles as "early break-in period" and go from there? You'll do great!

(Have you checked the air pressure in your tires?)

 

I had high expectations, having seen so many favorable first reports here.

The first numbers on our 2006 were over 50 mpg. We're actually seeing only mid 40s on that car now, which I attribute, in part, to the Nokian i3 tires I put on recently.

Tire pressures on the 2010 were initially low, at about 33 psi. And yes, I'm expecting better on the next tank!

 

Accurate, just not precise.

 

Re: 2010 calculated vs. computer MPG - Gas density according to temperature?

Just a guess: gasoline density changes with temperature.

Assuming the gas station tank is kept at 10°C (50°F), its density should be higher than the gas density in your car at ambient temperature (let say at 25°C [77°F] in the summer). You then have more gas (in volume) than what you bought. Does the Prius injection system account for this?

Note: Gas pumps are temperature compensated but I don't remember what reference temperature is used.
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If not, there would be an inherent difference between the volume of gas you buy at the pump and the volume of gas the car burns, hence a slightly lower displayed MPG than the calculated MPG...

In a cold winter the results would be reversed and you would get more displayed MPG that the calculated MPG (although that wouldn't show through the reduced efficiency in the winter...).

This would average on a whole year though…

Does that make any sense?

 

Is this 254 miles between your first and second refills?

If this was your first refill, I'd discard it as a less than full tank at delivery. Low numbers mean nothing until you (not the dealer) personally fill the tank for your second time.

 

396.7 miles / 10.2 gal = 38.8921; Computer said 41.2

The shame, the ignominy, the utter despair! How could my Prius let me down like this?
FYI - Eco mode, AC at 77 (extreme heat & humidity), tires at 38/37, all short trips, no EBH.

41.2 / 38.8921 = 1.0593, so almost a 6% difference, which seems pretty badly programmed to me.

Time to fly FireEngineer down to install an EBH and give me some driving lessons! (I'd call Dan Bryant, but he's not responding to e-mail).

 

First tank 57.1 comp. 55.9. Dealer filled the tank when I picked it
up to two click's, I filled to four clicks.
About the same diff. as my gen2 always +/- 1 to 2 mpg.

Better mileage then the gen ll pretty much the same route
and travel for 464 miles 60% highway 75mph with AC and AC in town. It's warm here in Tucson this time of year, may ll would avg. 50 mph

 

My first tank after finishing the tank the dealer filled. I went 305.8 miles on exactly 5.0 gallons for 61.2 mpg. The computer said 62.5 mpg, so not much error.