Gas Mileage: Whom Do You Trust?



Maybe so Dave, but I don't see them jumping to correct an issue when they're told about it either. 5 days went by before weights and measures guy finally got around to check one station. Three pumps on two islands were shut down. He was also reluctant to say how much the pumps were off because anyone with a receipt has a legal right to be compensated. I noticed self service stations are more likely to have pumps that are inaccurate. I wonder why. Could it be that blanket one year testing intervals are either too far apart or that volume should be the determining factor instead of time? There's also an issue with the flow back valves that are meant to keep the lines pressurized. When those valves don't work properly not only do you have to pay for the gas to fill the hose you lose out on the gas in the hose after the fill. The hose may contain as much as a coffee can full of gas. I know because I used to siphon gas when I was a kid. Instead of siphoning out it siphons back into their tank and the next customer has to pay to fill the hose again. Yeah, it pays for them to be woefully ignorant.
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Back in the late '50s I had a 125CC BSA Bantam. In those days the gas stations all closed around 10 p.m. I could get a tank of gas by draining the hoses at a couple of stations. :-)
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I had a quarter (25 cent piece) with a hole drilled in it and a first string guitar string attached to it. I cleaned out the soda machines by dropping it down the coin slot and pulling it back up over and over. I saw it done in a movie used on a pay phone. But what goes around comes around. When my stolen van was recovered the police said ever since the movie Terminator came out the ignitions on the columns were being smashed just like in the movie. A simple pull on the exposed metal bar started the engine. They say movies and TV don't influence kids. Yeah, right.
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I use the odometer/tank to track my overall MPG and the online display to handle specific segments or learning how to best drive my Prius. With just 10 months and a little over 14,000 miles, I'm running 51.4 MPG in an NHW11, 03 Prius. Given the EPA rating of 52/45 (City/Highway), I'm OK with the performance.
I find the MFD useful for specific, short term measurements but when it comes to what I plan by budget or trips, I use my Odometer/tank measured, MPG vs MPH chart:
http://hiwaay.net/~bzwilson/prius /
Bob Wilson
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Davoud wrote:

I would trust the display. It's tied to the very prcise fuel metering system that sends gas to the injectors. You have a big discrepency that may be attributable to some filling errors - e.g.: early pump shut off, car not level when filling, fuel coming from warm storage tanks.
While it doesn't make a noticable difference in a car that gets 50 mpg, it is worth noting that you burn fuel by weight, not volume.
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Well, my 92K mile experience would differ. While there are variabilities in individual fillups, over the span of 3 years where I logged each of 203 fillups and the number of miles between them and found that the MPG display in the Prius was 5.21% higher than the calculated MPG. It was pretty consistent; a few times higher and a few times lower. I have to believe that over 203 tankfulls at all sorts of gas stations, that the errors due to top offs, temperature changes, pump accuracy etc. should be pretty much netted out. At the end of the day, the true cost of fuel for the car is what I actually pay for it and pump into the car, and not what the meter in the car might tell me. Dividing that by the number of miles I drove as shown on the odometer will yield the true MPG.
Curtis CCR wrote:

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PriusGeek wrote:

Yes, the true cost of the fuel depends on what you pay at the pump - which has nothing whatsoever to do with the original question.
Everyone has their own opinion. After posting my pump tracking spreadsheet in the PriusChat forums, John1701 (a well-known Prius blogger) challenged that the only true measurement of MPG is the hand-calculated method, not the MFD, and suggested I view his 114k+ miles of records. I did so, plugged them into my own spreadsheet, and found out that his lifetime MPG were less than 2mpg off from hand to MFD.
While some Prius geeks (not talking about you, PriusGeek!) track fuel, mileage, cost, and MFD readings, I also happen to track fuel pips on the "guess" gauge. For the most part, the first pip disappeared after anywhere from 110-160 miles, and all the rest dropped after 20-60 miles. The one time I got GREAT mileage for me (56), the first pip dropped at 50, and the next tank showed crappy mileage (36). Translation: I didn't fill up all the way prior to the great mileage. Amazingly, my MFD said both tanks were about the same as far as MPG. You guys can debate this all day, talking about teh 9/10 of a cent issue, the fuel injector issue, the gas pump issue, and the "I know I filled it all the way up, even though I'm at a different gas station using a different model of pump. The only constant in this is the Prius and its MFD, so that's what I'm going with. Somehow, I think it's slightly more accurate than a commercial gas pump or any driver on the road, but then, I tend to trust computers. They're pretty good at that cipherin' stuff...
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Right. Your readings and calculations are valid for your car. Isn't there a site where a wide range of such readings and calculations have been posted? Damned analog devices. Hard to get consistency in mass production.
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No, not an analog issue (except for the fill-ups).
How come PriusGeek reports a 5.2% difference and I get about 3% for the same calculation? I have a 2005. Is the "classic Prius" different?
Does anyone know exactly how the MPG display is calculated? Is it the calculation of the total fuel through the injectors divided by the distance covered, or some kind of a weighted average?
Al
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I'm sure that individual driving styles as well as differences between vehicles (tire wear, brakes, etc.) accounts for some of the differences. I keep the same data on my '06, and over 16 tanks, my variance is 2.7% (display more optimistic than the calculated value) But that's not the whole story, as I have several tankfulls where the variance was as much as +/- 10%, so it is important to look at the data over a significant number of fillups.
On the other hand, my 02 Classic was the best car I ever owned until I bought my 06! For more perspective I get infinite pleasure filling up with 10 gallons after 500 miles of travel while the Hummer next to me is maxing out his credit card after a couple of hundred miles.
A Sherman wrote:

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That's the analog part Al. Open the injector and some fuel squirts in. How much? Well, that depends on fuel pump pressure and how long the injector is open. Yes, you can count the times you open the injector and go digital all the way to the display, but the part that counts (pun intended) is analog.
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Bill wrote:

Yes, it's called GreenHybrid.com. Currently there are 2,379 cars in the mileage database, and the majority of them (773) are Prius II's. Each user can decide if they want to record by hand or MFD.
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