ISRocker

iTrader: (1)

I was just wondering what is the best way to calculate mpg. The computer in the car says "Tank Average 22.4". But when I calculate it by using the odometer reading and number of gallons to fill up the tank I get 20.96 mpg. Not a huge deal, but just wondering which way is more accurate...

Jeff Lange

iTrader: (1)

Fill up, set trip meter to 0. Drive, drive, drive. Fill up, record gallons, record miles.

If you want accuracy, fill at the same pump. Quite a bit more accurate than the computer.

Jeff

ISRocker

iTrader: (1)

OK, I did a search and found my answer in another thread...Sorry about that.

KillaIS250

iTrader: (1)

For iPhone users, you can download the app "Road Trip" There's a free and a paid version. The free version is really enough. Every time you fill gas you just have to input the gallons filled, $ amount, and price of a gallon along with your mileage. From there it does all the hard work.

quick123

iTrader: (2)

Easiest way is to reset trip A or B when you fill up...when you add gas the next time, just divide the miles on trip A by the gallons you put in shown on the pump...

Duke02

iTrader: (2)

I've wondered about this before. I think consensus is that the computer's calc isn't ideal, but the pumped gallons / miles driven seems to have a greater chance for error to me for the following reason - As far as I understand, the pump click off mechanism is triggered by pressure, which opens the calculation to error based on mechanical calibration differences between 2 pumps or even shift in that setting at the same pump over time. As an example, my wife's G35 clicks off all time before it's full, at varying levels of fullness - a design issue with the G. I guess my point is that the actual fill level could vary enough to make it hard to know how much gas was actually consumed.

I may be completely off on that so feel free to correct me if I am, it just seems that a electronically controlled fuel injected car would have a more accurate ability to know how much fuel was consumed than a mechanical switch at a gas station

mclovintsx

Doesn't the computer calculate it based on the cumulative mpg overall and not just the specific one gas tank?

Duke02

iTrader: (2)

There's two MPG displays. One is cumulative since you last reset it and one is the current tank. At least that's how it is on my 06. Not sure if they changed it

Jeff Lange

iTrader: (1)

It is still the same.

Jeff

paulv7260

So, does the ISx50 calculate avg MPG based on actual consumption and distance, much like the formula we do in our head, or does it get down into sensors, rpm's, throttle position, gear position, ECT mode and all that?

Just curious. I've only had my IS350 for 2+ months now and I've noticed that the readings are creeping higher even though I've been known to mash on the pedal on occasion (not complaining, just saying. )

kingster

Don't brake your head. Just fill up pay and enjoy your ride. You will be notified when it is time for a refill.
Beware! The next time at the pump you may end up paying more, much more at the pump.

XSV

iTrader: (1)

What I do is reset Trip A or B when I fill up a full tank. Then when it's time to refill, I just divide the number of miles displayed on Trip A or B (which ever one you reset on your last fill) by the number of gallons purchased. I am getting around 28mpg with highyway and city.

carlos3366

you are doing it right! i would think average (22.4 + 20.96)/2 = 21.5 is closer to what you are getting.

Bichon

It doesn't matter that the pump clicks off at slightly different degrees of fullness, provided that you average your results over several tankfuls. Over time, the instances when the pump clicked off a few tenths of a gallon earlier than the previous tankful will be counterbalanced by the ensuing instances when it clicks off later.

As for the accuracy of the trip computer MPG display, the engine management system controls fuel injection by varying the injector pulse width, and in turn, how long the valve in the fuel injector is open. The ECU is very good at adjusting the pulse width of the injectors to make sure the engine isn't too rich or too lean (based on fuel map lookups in the ECU, and adjusted in response to feedback from the oxygen sensors). Converting injector pulse data into a measurement of the absolute quantity of fuel consumed is done after the fact by trip computer, and is not 100% accurate.

I believe that the manual calculation, especially if averaged over multiple tanks, is more accurate.