why estimate mpgs?

[jamarimutt]

Many Prius owners use the old method of estimating mpgs; namely, fill the tank, see how many miles it lasts, divide miles by gallons...

If the Prius knows how much gas it injects into the engine and how many miles it has travelled, what's the point of estimating the old way? Seems to me that it's impossible to be more precise than the car's calculation. Any thoughts on this?

José

[richard schumacher]

Many Prius owners use the old method of estimating mpgs; namely, fill the tank, see how many miles it lasts, divide miles by gallons...

If the Prius knows how much gas it injects into the engine and how many miles it has travelled, what's the point of estimating the old way? Seems to me that it's impossible to be more precise than the car's calculation. Any thoughts on this?

It's the *car* doing the estimating, relatively speaking. The car knows very well how far it's travelled, but it doesn't know very well how much gas it's used. The gas pump is more accurate than is the car's fuel-flow meter, and much more accurate than the car's gas gauge (witness all the grumping about the effect of the gas tank bladder).

[rflagg]

Originally posted by richard schumacher

It's the *car* doing the estimating, relatively speaking. The car knows very well how far it's travelled, but it doesn't know very well how much gas it's used. The gas pump is more accurate than is the car's fuel-flow meter, and much more accurate than the car's gas gauge (witness all the grumping about the effect of the gas tank bladder).

Has it been proven that the car's fuel flow meter isn't accurate? Also, when you calculate your mileage, do you figure in wind resistance (which can sometimes make a major improvement/dent), or temperature, or your height above sea level, or anything else? Does the Prius, and where's the proof of what exactly the Prius uses for estimating MPG on the fly / over the tank?

I know the pumps are supposedly regulated by the government, but that doesn't give me much hope for those pumps either - who's to say they aren't miscalculating - before you fill up, do you check with the gas station to make sure your specific pump was recently checked by the government and is within standards?

Food for thought.

-m.

[jamarimutt]

I gather that the Prius measures the amount of fuel injected into cylinder 1 and multiplies that by four to determine total amount of fuel used by the car in a period of time. Miles travelled (the car knows how many) divided by the amount of fuel used during that time (the car knows pretty closely) provides an estimate seemingly much more accurate than anything obtainable by the old method (and only method available for other cars).

I don't see the point of estimating mpg any other way, aside from just being used to the old method. Reminds me of the time when the first calculators beame available in the mid 70s. People would add with the calculator and then do it manually to double-check.

[Smooth Operator]

Originally posted by rflagg


I know the pumps are supposedly regulated by the government, but that doesn't give me much hope for those pumps either - who's to say they aren't miscalculating - before you fill up, do you check with the gas station to make sure your specific pump was recently checked by the government and is within standards?

Food for thought.

-m.

Ahh, and how do we know it is really gasoline in those pumps?

My Prius consistently provides a more optomistic estimate of my fuel efficiency than does my calculation based on distance travelled and fuel pump reports. This is regardless of which gas station I happen to go to for my fuel.

I understand that some other people have pretty consistently pessimistic estimates from their Prius.

[egs]

If anything, I would expect the Prius calculated MPG to be more accurate since it's measuring the actual flow of fuel.

Because of the bladder and inconsistency of when the pump shuts off, wouldn't the manual calculation of MPG be more error prone?

[notforyou23]

I understand that some other people have pretty consistently pessimistic estimates from their Prius.

Is that true? Because I too have gotten a number of 'pessimistic' estimates from my Prius. I just tossed it up to my poor math skills (even though I use a calculator!!)

I now just do a quick average of the two and figure that's gotta be pretty close. Either way, I'm getting at least double the MPG of anyone else I know!!

PeAcE

[EssD]

The Prius does not measure the amount of fuel delivered by the #1 injector. It measures the number of times the injector fires and multiplies the firing count by a fixed estimate of the amount of fuel injected in each firing. The actual amount of fuel injected in each firing can vary based on temperature, engine load, etc.

[Tempus]

As mentioned previously, technically the Computer is the one 'estimating' the mileage.

I am 'calculating' the mileage when I divide miles travelled by gallons used.

The bladder and gas pump cutoff variance can make my tank to tank calculations less accurate, but averaged over time my mileage is most certainly most accurately determined by acutal miles and actual gallons.

[plusaf]

wow... my head is spinning....

if i note how much gas went through my car and how many miles it carried me, where are the errors? 1) the gas pump is not measuring right. yeah, could be, but there are specified limits for its accuracy. if you don't trust the accuracy, complain to the folks responsible for the calibration. 2) the car's odometer is off. tire inflation, tire changes? wait a minute! does the car interact with the GPS for odometer calibration? did i read that here, somewhere?

hmmmm.. odometer can't be that far off.

so, TOTAL MILES on the odometer divided by TOTAL GALLONS that went into the tank SHOULD be pretty darned accurate, and more so the longer you do the computation: total miles and total gallons, versus tank-by-tank! heck, i can squeeze about 1.2 gallons into my Prius' tank after the nozzle first shuts off. if you don't fill up with the same pump at the same station, and only to the first nozzle cutoff, you're adding many more variables to the fuel mileage "estimate".

and hold on a second....

the car is calculating fuel consumption by the NUMBER of injections?! help me if i'm way off base here, but don't most fuel injected engines vary the DURATION of the injection?!!? how else can the computer maintain an air-fuel ratio control when the amount of air going through the engine varies with rpm, load, temperature, etc., etc., etc?! well, that IS how it does it, so counting injections or multiplying by four (or fourteen) makes no sense, since it's frequency (or number of injections) TIMES duration that creates the "quantity of fuel burned."