370Z computer consistently overestimates MPG
 

I have a 2010 370Z bought almost 1 year ago. I save my gas fill-up information at Fuelly.com. For the first few months the Fuelly MPG calculations matched the Z computer calculations. Now the Z computer consistently overestimates the MPG by .5 to 1 MPG. Anyone else seen this?

Will

The computer estimate is just that... an estimate. The only way to accurately calculate your mileage is the old fashioned way. :tup:

its constantly adjusting to you driving style,

You also have to take into consideration that as your tires wear down, the numbers will change. As the tires wear the travel distance for one revolution is less than it was when they were brand new.

Not sure tire wear would explain it. Tire wear would affect the odometer in both cases (computer calculation and owner calculation). The variable is how much perceived consumption is occurring. More likely is that the tank doesn't fill to the same level every time. Every pump is different and the filling trigger may go off at different tank levels.

For example, say you drove 350 miles on a tank of gas. If you fill up and get 17 gallons in the tank, your MPG is 350/17=20.6mpg. Now say a different pump was used and put in 17.4 gallons (or you topped it off a little). The same calculation gives 350/17.4=20.1mpg. There's a 0.5mpg difference for ya.

according to my calculations, i only get about 15 mpg :)

miles driven/gallons consumed = 15 mpg

driven mostly in the city.

Hey Spear, wouldn't the onboard estimate be able to calculate based on fuel being used? I know if I was the person who set it up, I would pull info from the ecu to get as precise an estimate as possible. That way it wouldn't matter if the tank got filled half way or all the way to the brim. I'm just wondering here. Maybe somebody will know how the onboard estimate is calculated.

^The discrepancy is more in the manual calculation when you try to figure out how much you put in and burn. The car does read from fuel flow (I think), so it's probably the more reliable of the two methods.

I have an 09 and my computer has been UNDER-ESTIMATING w/in .5-1.0. I'm getting better MPG's than the computer states and thats better for me.

+ how much you eat/weigh. :icon17:

in the end, gas mileage isn't a big concern for the both of us i think.
according to my calculations, i am going to fill up the gas tank twice or three times in 2011 based upon my driving habit. i filled up the gas tank at the beginning of the year and have only driven 90 miles to date :)