Toyota Prius Forum banner

1 - 18 of 18 Posts

·
Registered
Joined
·
24 Posts
Discussion Starter · #1 ·
First off let me apologize for making a separate post on this subject. I thought it would warrant several “specific” comments to the subject.

I’d like to know how many of you Prius owners out there find that there is an error between the “Average MPG” displayed on the dash screen at the time you fill up and the “Actual MPG” calculated by dividing miles traveled by the amount of gas used in that many miles. And how much of an error.

I find that there is a fairly big difference in my case. It is not always consistant as well. Some times it’s above and at times below the calculated number.
Yes I’m sure that when filling up that I stop pumping at same point (as much as possible).
I also fill up at stations that are mostly on level ground.

I’ve checked my speedometer and odometer with my GPS and find that both are quite accurate.

BTW I suspect that may be some of the reason that there is so much being said about MPG errors. Most American ( Chrysler) and Toyota’s (Camry & Avalon) that I’ve owned have 2 – 5 MPH error in their speedometers. The Prius I have is the only car I’ve had in 20 years that has an accurate Speedo.
 

·
Registered
Joined
·
1,560 Posts
Observations,

MPG discrepancies have been discussed at length. to summarize... some folks see about a 3% difference between MFD and tank when measured over multiple tanks. Because of variables in fuel pumps, bladder elasticity, temperature, mood, and cosmic rays...the only way to measure is over multiple (as in more than 5) tanks.

As to the speedometer... I regularly pass a radar-sign in a construction zone.. at 55 the sign says 54. At 70 (shh..don't tell) the sign says 68... so ther is some discrepancy, though minor. YMMV
 

·
Registered
Joined
·
581 Posts
I have noted the same variations between the computer's 'average' and my 'calculated' mpg as well.

It has been noted on these forums that most think it has to do with the flexible bladder and the fact that 2% of `50mpg reveals a larger number than 2% of say 20mpg as a reference.

Several people will, no doubt, correct me. 8)
 

·
Registered
Joined
·
669 Posts
If you are interested in accurately tracking RELATIVE MPG, the car's calculation is as consistent as possible (it's based on the fuel injector). It's hard to get a consistent fill tank to tank in ANY car, and much more so with this one. While I'm sorta interested in the absolute number (OK, OK, I'm dying to manage a tank average of 50+ and I've hit just short a few times) I'm MORE interested in whether I'm doing better now than I was when I first bought the car, etc. As far as I know, you can trust the car's calculation to answer that sort of question.
 

·
Registered
Joined
·
111 Posts
When I graph my miles per gallon, [as calculated from fuel used] the line varies up and down from tank to tank, but the average is almost a straight line.

This is because of the vagaries of the bladder, not the MPG as calculated by the car.
 

·
Premium Member
Joined
·
2,815 Posts
http://autos.groups.yahoo.com/group/toy ... ?id=444783

On average, what is the difference between your mpg calculated by the car's computer and by the gas station fill-up method?

Choices Votes %
computer 5mpg or more higher 16 3
computer 4mpg higher 38 7
computer 3mpg higher 68 14
computer 2mpg higher 105 21
computer 1mpg higher 11 2
both methods about the same 124 25
computer 1mpg lower 4 0
computer 2mpg lower 8 1
computer 3mpg lower 15 3
computer 4mpg lower 4 0
computer 5mpg or more lower 6 1
varies too much to give meaningful average 81 16
 

·
Registered
Joined
·
581 Posts
Here is my data sheet. Exclusive use of the same gas station and pump since 15 July 2005. I also use cc and drive the speed limit and drive the same route.
 

Attachments

·
Registered
Joined
·
24 Posts
Discussion Starter · #8 ·
MPG

Thanks everyone.

I guess I'll go by my calculations and not the display average.

I fill every time I get gas and record every time.

Over time it does give good indicator of total.

I also find cold makes it go down and warmer goes higher.

Good to get other input's
 

·
Registered
Joined
·
248 Posts
Mine vary by about 3-4% but slowly the life time mpg is converging on the average of the MFD displayed for each tank.
 

·
Registered
Joined
·
60 Posts
I didn't do this table, I found it on another forum.
I thought it is interesting how much the MPG changes with temp and speed.


<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
MPG TABLE PER TEMP RANGE & FUEL
20ºF COLUMN USES 10% ETHANOL FUEL.........(110,500 BTU/Gal US)
40ºF - 60ºF COLUMNS USE WINTER FUEL.......(112,500 BTU/Gal US)
80ºF - 100ºF COLUMNS USE SUMMER FUEL..... (114,500 BTU/Gal US)

Speed....20ºF......40ºF.......60ºF.......80ºF......90ºF.....100ºF
38......53.52.....58.92......64.50......71.50.....69.99.....65.70
45......51.66.....56.75......61.62......68.51.....68.04.....64.87
50......49.71.....54.50......59.04......65.54.....65.47.....63.06
55......47.40.....51.89......56.08......62.08.....62.35.....60.50
60......45.08.....49.26......53.12......58.61.....59.15.....57.79
65......42.39.....46.26......49.75......54.77.....55.46.....54.49
70......39.82.....43.37......46.55......51.10.....51.89.....51.25
75......37.39.....40.65......43.55......47.71.....48.52.....48.11
80......35.10.....38.10......40.75......44.53.....45.36.....45.15

The above table represent days without rain, snow or wind. There is
almost always at least 3-5 MPH winds in one direction or another so,
allow me to show what a 5 MPH headwind and 5 MPH tailwind do to some
of these figures.

<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
 

·
Registered
Joined
·
7,161 Posts
I have had a discrepency of about 2 MPG where calculated is 2 MPG less than MFD. MFD is good for short term variations and comparisons, but what you put in versus what you travel (calculated) will show more of a true value over several tanks. For one, it accounts for evaporative losses that even if sent to the intake manifold for environmental reasons, bypasses the injectors. That's how I can explain higher MPGs on the MFD on a recent fill, and at a low tank, especially in warm weather.

As for speed, just checked my speedometer against a radar trailer, and it is accurate until I get within 100 feet of the trailer where the radar reading starts dropping to 2 MPH. This is understandable as it starts reading at an angle.

As an interesting asside, the road the trailer is on is supposedly a short 40MPH zone near a relatively new golf community, and when I inadvertently drove 44, it blinked 44, yet the speed limit shown above the trailer showed 45. Yesterday I drove 40 and it wasn't blinking, but still showed a 45 limit. This is a county line road where the trailer is on the pasco side. The strange thing is, the Hernando side speed limit is 50.
What really ticks me off is the 40MPH zone is a divided road with 2 lanes on each side, and where the road becomes undivided with 1 lane on each side, the Pasco side is 45MPH.
 

·
Registered
Joined
·
1,028 Posts
Re: MPG

larry31 said:
I guess I'll go by my calculations and not the display average.
I fill every time I get gas and record every time.
Over time it does give good indicator of total.
I also find cold makes it go down and warmer goes higher.
Good to get other input's
The display average is completely accurate. The problem with doing it manually is that the number of gallons you put in at a fillup is *not* the same as the number of gallons you burned since the last fillup. Thus the MPG calculated as (gallons in this fillup)/(miles since last fillup) is not very meaningful.

The discrepancy *does* average out over many fillups. But even so you can get a more accurate result by recording the MFD MPG and odometer reading at each fillup. Those data allow you to calculate the lifetime MPG, and MPGs for any interval between any number of fillups.
 

·
Registered
Joined
·
7,161 Posts
The display is precise, but not necessarily accurate. If it were accurate, can you explain what happened to the 22 gallons I lost since I bought the car?

After 41809 miles, I put in 756.47 gallons, yet going by the display I only burned 734.86 gallons.
 

·
Registered
Joined
·
1,560 Posts
DanMan32 said:
The display is precise, but not necessarily accurate. If it were accurate, can you explain what happened to the 22 gallons I lost since I bought the car?

After 41809 miles, I put in 756.47 gallons, yet going by the display I only burned 734.86 gallons.
Hep's been juicing up his coffee at your expense ;-)
 

·
Registered
Joined
·
85 Posts
Not to go completely off topic, but

I too have have been questioning how accurate my Speedometer was. The bridge I drive over everyday has a speed reading meter and I have noticed that it is usually off about 5 MPH slower from what my car says. this is consistent everyday and I have varie dmy speed to test it and everytime 5MPh off. I am not too concerned, but thought it was interesting. Of course maybe its not my speedometer, but the speed reading meter that is off.[/quote]
 

·
Registered
Joined
·
1,028 Posts
DanMan32 said:
The display is precise, but not necessarily accurate. If it were accurate, can you explain what happened to the 22 gallons I lost since I bought the car?

After 41809 miles, I put in 756.47 gallons, yet going by the display I only burned 734.86 gallons.
True, accuracy is not precision. To be fair, you don't know that you put in 756.47 gallons; you know that that is what the gas pumps say you put in. Curiously enough the discrepancy is in the gas stations' favor. And of course that net -2.9% is a lifetime error. The error of a manual calculation on any one tank is likely to be considerably larger, and either + or -. The MFD should at least be consistent since it isn't sensivive to any variations between different gas pumps or variability in the gas tank.

I don't know the source(s) of that -2.9% disagreement. One contributor is evaporative losses. I don't know how much gasoline might evaporate out of a Prius tank over three years. If it were 22 gallons (I doubt it's nearly that much) then the gas pumps and the MFD could both be accurate.
 

·
Registered
Joined
·
669 Posts
DanMan32 said:
The display is precise, but not necessarily accurate.
I dunno if it is really all that precise, either, just 1 sig fig after the decimal. :D I will repeat, though, that what it is is CONSISTENT which the manual calculation is NOT. If I understand the system correctly, the limiting factor on how closely the displayed mileage matches the actual is the variation on the performance of your specific fuel injector from the average. That's assuming the ODO has less variation from truth and that the count of fuel injector cycles is accurate.
 

·
Registered
Joined
·
581 Posts
I have noted differences between the AVERAGE MPG display ODOMETER and the dash ODOMETER, sometimes as much as 2 miles difference.
 
1 - 18 of 18 Posts
Top