Is the computer really that innacurate?

Well, Wolfman, that looks like about a 10% difference, more or less. Oddly enough, that's just about the differential between the mpg I calculated on my Navigator and what the computer on the vehicle is telling me. I'm actually getting 10-11 mpg while the computer insists that it's more like 12. Of course, with those kind of numbers, I suppose it doesn't really matter a whole hell of a lot.

 

Wasn't aware they worked that way.

I thought (erroneously, I guess) that the computer somehow used the gas gauge and odometer....sort of using the amount of gas it "thinks" has been used and then dividing the miles travelled. :? :?

 

I'd be really surprised if the Prius has a flow meter. A precision flow meter would be very expensive and still would have some amount of error. If there is a flow meter it's probably not much better than +/- 10%. It's probably either using the sensors in the fuel tank to infer mpg or more likely some complex calculation based on an "average" engine performance map. It does know when the engine is running, at what RPM, throttle opening, etc and of course the miles driven. I'm surprised it does as good as it does at calculating mileage. On second though, maybe it does have a cheap flow meter after all. I think it's generally doing much better than 10% for most, probably more like 5%.

But if the computer does know how much fuel is injected into the cylinders (using a flow meter) it is as simple as dividing that into the miles driven during the time that injected fuel was consumed. None of the RPM data, efficiency data, what gets converted to electricity, what goes to the electric motor, etc is really needed for this simple mpg calculation.

 

Good point Wolfman. In that case 10% does appear to be a big difference. Do you think certain types of driving might affect it more? Many posts I've seen state that it's been conservative by maybe 2 mpg or so. I know I'm wondering about if I should top off and if I do I have to be consistant with it. Some say it will take a gallon or so consistantly but I'm sure that is temperature dependent. Maybe we should start a poll about the difference between the predicted and actual to see how bad it can get?

 

Maybe in time the membership will increase. I like the fomat a lot better than over at yahoo. And you can't post pics over there because they have so many already. And by then they'll be more Priius owners too. I just decided to start posting here to help get things going instead of just reading posts. The RX-8 has a site similar to this one and it's very very active.

 

i think i will accept a 10% margin


there is no way to monitor the exact octane/purity or measurement of gas bought on the street. one "tank" of gas cannot be equal to another "tank" of gas, even if from the same station on the same day. there has got to be at least a slight difference between the top, middle and bottom of the stations storage tank. did the guy before you double trigger and drain the hose (that is isually good for a couple of ounces),.

there is no true way (outside of a controlled environment) to compare two tanks to the degree of many people/s comparisons. 5%-10% sounds good to me


just an opinion

sid

 

Polls

Sensing the thread drift here re POLLS, I'll jump on the bandwagon.

Since, as Wolfman stated, there is not enough traffic here to support a poll at this point in time, it might be interesting to see what would happen if a member posting a poll on this board also posted a link in the Yahoo group to the poll.

If someone comes up with a decent poll (such as the one suggested by FredWB), I say post the poll here and then a link to the poll in the Yahoo group.

Just my .01653 Euros

 

Just a thought on mpg -- I started logging the gallons bought, dollars paid, and miles driven from the first tank when I drove it off the lot on 10/27/03 with 4 miles on the odometer. I now have about 5600. My theory is the # of gallons I buy is known, and the # of miles I drive is known, and when I get it into a spreadsheet I'll have a very accurate calculation of lifetime mpg, regardless how full I get the tank on each fill up. I'm getting about 45 mpg, but I had the fun of seeing it average over 70 mpg on a downhill stretch in the Ozarks last Thanksgiving weekend. Ellen

 

Odometer accuracy?

I don't know how much of a difference it makes, but the accuracy of the computer is at least based on the inputs, i.e. how accurately receives data on the amount of fuel and the mileage (which I assume (?) corresponds to the odometer. I know the mileage can very SOME based on tire inflation, but 10% seems like a lot.

It might be worth at least checking the accuracy of the odometer by driving, say 10 miles on the freeway and watching the little mileage signs to the right (at least we have these in CA) to see if some (most/all?) of the error may be due to that.