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Discussion Starter · #1 ·
Just want to check: Just got my car a few weeks ago
I've filled up 4 times and have put about 1,050 miles on the car
4.29 gallons for 150 miles
6.86 gallons for 300 miles
8.03 gallons for 318 miles
8.37 gallons for 340 miles

My mpg were:
35.0
43.7
39.6
40.6 averaging 39.7.

I've trying to feather or coast as much as I can. My mpg on the display said 44.0 when I filled up tonight, but as you can see I only got 40.6

Is this normal or low?
Thanks.
 

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Don't worry about it. I live in a mild climate but I saw my first 50mpg tank in March after 5000 miles and 4 months
 

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I've filled up twice and am nearing 1000 miles. The first was 36mpg and the second 43. This time around looks like it will be about 43 again. I have short commutes and Chicago weather. Yesterday on a longer trip I tried the coasting to bring up mileage and realized that expressway driving is very unpredictable. Maybe I'll never see 50mpg, but hey, my Jeep was getting 12 in town, so I've nearly quadrupled my mileage. I'm happy.
 

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The single biggest factor in determining mpg is the length of your drive: You'll never average 50 mpg if your average trip is short. After this, the two most important mpg factors are weather (the Prius loves heat, dislikes cold) and driving style (the Prius loves steadiness, dislikes rapid changes in speed, either faster or slower). Some Prius owners are fanatical about maintaining correct tire pressure. In short, the Prius is sensitive to all the things that affect mpg on a traditional car. The main difference is that the effects appear magnified on a Prius because 10 percent of 55 mpg is a lot more than 10 percent of 20 mpg.

Contrary to Prius myth, the vast majority of drivers get better mileage on the highway than in town. Though the hybrid drive makes it POSSIBLE to get better in-town mileage - the EPA was surely being honest - in most real-world traffic conditions, this is not the case. My wife and I routinely average about 15 mpg better on the highway than in town.

In conclusion: Your particular situation is going to determine your overall mileage.
 

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Ah, well I get 58-60 MPG driving on US19 (avg speed 47, avg distance between stops due to traffic lights, 3 miles), but I decided to drive on the Suncost/Veterans yesterday. When driving 60, I got about 47-52 MPG, driving 70 got closer to 45 and under. Dropped my tank MPG of 60 at 180 miles to 56 at 240 miles.


So yes, you do get better mileage in city since your speeds are lower and your trips are long enough.

First 5 minutes, usually consumption screen shows 25-35 MPG. Then it picks up to 50+ with avg speed 45.
 

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drtravel47 said:
J I've trying to feather or coast as much as I can. My mpg on the display said 44.0 when I filled up tonight, but as you can see I only got 40.6
I'm inclined to believe the number on the display over any given fill because it is hard to get identical fill each time. I don't know what the statisitical variation in injector performance is, but that seems inherently more consistent than the fill-to-fill method. I guess I'll ask the assembled wisdom if it is normal to see a consistent difference between the two as large as this?
 

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drtravel47 said:
said 44.0 when I filled up tonight, but as you can see I only got 40.6
You got 44.0, not 40.6. On the Prius, the mpg computer is accurate. Traditional calculations are not.

Read this excellent article by efusco:

http://priuschat.com/forums/kb.php?mode ... 4f8686cab2

After you read it and digest it, you can decide to become an mpg junkie or, ignore most of it and drive your car safely, moderately and enjoy the heck out of a great car.
 

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DanMan32 said:
Ah, well I get 58-60 MPG driving on US19 (avg speed 47, avg distance between stops due to traffic lights, 3 miles), but I decided to drive on the Suncost/Veterans yesterday. When driving 60, I got about 47-52 MPG, driving 70 got closer to 45 and under. Dropped my tank MPG of 60 at 180 miles to 56 at 240 miles.

So yes, you do get better mileage in city since your speeds are lower and your trips are long enough.

First 5 minutes, usually consumption screen shows 25-35 MPG. Then it picks up to 50+ with avg speed 45.
Your situation is exactly why I say it's POSSIBLE to get better mileage in the city. My situation is very different: If I drive the seven-tenths of a mile to work - which I rarely do, since the walk is both short and pleasant - I have four 90-degree turns, four stop signs, one traffic light that's red 80 percent of the time, enough traffic to cause me to stop frequently at one or both of the two unregulated intersections, and two steep hills, both of which have stop signs in the middle. The only time I've seen even 25 mpg on this stretch is when the traffic light was green, which lets me off having to start from 0 at the bottom of a hill. So city mileage is extremely variable because city conditions are extremely variable.

I'm surprised you get such low mileage at 60 mph. Perhaps I get a benefit from the thin, dry Colorado air.
 

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Astrowoman said:
drtravel47 said:
J I've trying to feather or coast as much as I can. My mpg on the display said 44.0 when I filled up tonight, but as you can see I only got 40.6
I'm inclined to believe the number on the display over any given fill because it is hard to get identical fill each time. I don't know what the statisitical variation in injector performance is, but that seems inherently more consistent than the fill-to-fill method. I guess I'll ask the assembled wisdom if it is normal to see a consistent difference between the two as large as this?
Astrowoman is right: The display is undoubtedly more accurate for any single fill. I have no idea whether it's more accurate over a series of fills, during which individual fill percentages even out. But I agree with those who suggest we should be wary of regarding the gas-station-volume-calculated mpg as the "real" mpg. Gas station measurements could also be inaccurate - especially since money is involved.
 

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There is this poll still running on the toyota-prius yahoogroup:
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 36 8
computer 3mpg higher 65 14
computer 2mpg higher 100 22
computer 1mpg higher 11 2
both methods about the same 111 24
computer 1mpg lower 3 0
computer 2mpg lower 8 1
computer 3mpg lower 13 2
computer 4mpg lower 4 0
computer 5mpg or more lower 6 1
varies too much to give meaningful average 75 16


As of my last fillup on my 2001, my consumption screen odometer was showing 47.9MPG over the last 33963 miles since last reset (darn service people resetting my screen on me on a 5 month old car...). Calculating the MPG over the 38314 miles my car had gone, dividing by the 835.413 gallons I've used, gives me 45.9MPG for the lifetime of the car. So, yes, my consumption screen MPG is missing a few months of cold New England winter, but that's not enough for the ~2MPG difference...
 

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Here's an experiment: calculate the # of gallons used using mileage driven, and MPG on the screen. Add that amount, plus .1gal for roundoff errors and such. If your miles per tank slowly drip away, MFD is more optimistic than actual.
 

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Discussion Starter · #13 ·
not sure why my calculations at the pump wouldn't be accurate

1)I check it when the pump clicks off; I never go past the click

2) The survey posted noted that overwhelmingly the display was 2mpg higher than trad. calculations.
 

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drtravel47 said:
not sure why my calculations at the pump wouldn't be accurate
A long term average should be fairly good but you need to factor in errors in the fill and the odometer. The car's caluclation is based on fuel injector cycles and should be very reproducible even if the absolute calibration may be slightly off. It is certainly better for comparison.
1)I check it when the pump clicks off; I never go past the click
If you always use the same pump, it may be fairly reproducible but different pumps will have different behavior, and I'll wager the ambient temperature probably affects it as would any maintenance of the pump. Topping off would make it even less reliable (but potentially more accurate if you could really guarantee the tank was FULL each time.)

2) The survey posted noted that overwhelmingly the display was 2mpg higher than trad. calculations.
It actually peaked at "about the same" (24%) but there appears to be a bias towards the high end. I wonder if there is any difference between the classic and new models. At any rate, the fact that there is a bias towards the car underestimating the mileage is interesting.
 

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drtravel47 said:
not sure why my calculations at the pump wouldn't be accurate

1)I check it when the pump clicks off; I never go past the click

2) The survey posted noted that overwhelmingly the display was 2mpg higher than trad. calculations.
A question and an observation:

Question: Have you personally measured the gasoline that comes out of the pump? Is what the pump calls a gallon exactly a gallon? Unless you have, you can't be certain that the pump calculation is inherently more accurate than the screen calculation. The fact that many Prius owners report that the screen calculation is a little high may merely be evidence of widespread inaccuracy at gas pumps - an inaccuracy that would increase gas station profits. (Insert ominous music here.:wink:)

(Digression: Because we've seen these pump numbers for our entire lives, we tend to assume they're accurate. When the Prius gives us contrary numbers, we tend to assume they're inaccurate. This is a natural human psychological trait. In law, it's long been recognized that the prosecution has an enormous advantage, because it presents its case first, and people tend to believe the first version they hear of a story. The defense has the much more difficult task of challenging an accepted viewpoint.)

Observation: The Prius' gas bladder is notoriously finicky. In cold weather it stiffens, which means less gas fits in the tank. As it changes capacity, individual fill numbers can vary widely.

For instance, we had a recent tank that was 50 mpg on the screen, but 53 according to the pump; the next tank was 55 on the screen but 49 according to the pump: I had a bad fill followed by a good fill. This is why, even if the Prius' screen isn't perfect, it may be more accurate for any given fill.
 

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Discussion Starter · #16 ·
good point, maybe I should stop bringing a calculator to the gas station.

I probably won't....it's an ingrained habit....like I HAVE to balance my check book. I think having the cool geek car fits into this personality group. I already created a spreadsheet to check how much money I'm saving from my Mercedes, which I sold to buy the Prius. Of course, it's supposed to 80 here today, for the first time....I miss the convertible. Guess we'll have to wait for the Prius Cabriolet.
 

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Odometer inaccuracy should not be a factor. Both the MFD and the calculator use the same odometer, so that cancels out.

Variations in tank fill should level out over several tanks. I'd say a minimum of 5 tanks, unless there is a gradual reduction in tank fill, in which case eventually there will be no gas in the tank.

I would agree in the possibility of a problem with pumps measuring accurately. A 5 gallon gas can would take care of that problem. You might want to calibarate the can though, and clearly mark the gallon intervals with indelible pen on the outside.
 
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