Trip Computer- MPG Estimate vs. Actual

davidyan

So I was bored and decided to create a spreadsheet to track the MPG I get everytime after fill up. Driving 333.3 miles since the last FULL fill up, I used 15.371 gallons equating to 21.7 MPG.

My trip computer read out an average of 21.1 MPG. I made sure it was reset immediately after I filled up last. This is a difference of about 3% between the computer reading and my actual calc. Does anyone think that this is significant?

Couple plausible explanations for the difference:

- The car's computer calculated that I used more than 15.371 gallons as per the gas station meter. If this is the case, I'll make sure to continue buying gas from that station at that pump...
- The gas pump could have stopped a little short of a full fillup.
- My previous tank could have been slightly overfilled and not again this time.

Its unlikely that the an inaccuracy in the odometer reading of 333.3 miles could be the culprit since the car's trip computer probably uses the same readout for its calculation, or something mathematically tied to it like avg. speed.

Sorry for the otherwise random post but I thought I'd share and will continue to track until I get tired of it.

VCuomo

The 3% variance is probably the variance in how much you filled the tank at each fill up. The only way to accurately compare manually from one fill up to another is to fill the tank up to exactly the same level each time. Pretty hard to do on the Z since you can't really see the fuel level down in the tank.

I'd trust the computer.

FricFrac

I'd go with the computer as well.... others have claimed it to be accurate to .1 MPG... you'd have to check their method to see if the claims are valid or not but I suspect the computer is fairly accurate....

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need4speed

Lol. 15.73 gallons is getting u 333. Somethin miles an hour.
I think that's good enough info. If u think about it different things will
Change that constantly such as. Are u driving on inclines. Making more left
Turns.
Are there more impuritities in this weeks batch of gas vs last week.
City vs highway ratios will change.
Its almost pointless unless you are getting to little mileage below its range.
And vq engines after around 5k to 7k miles. Increase in fuel economy
Drastically. You will probably see 23 to 24 combined in a few months.

VCuomo

I don't think you understand... He's not saying that the MPG varies from tank to tank (which, as you point out, it will), he's saying that if he manually computes the MPG it will be different from what the car's computer says the MPG is for the same tank of gas.

need4speed

I feel you... I kinda see what u mean. He's trying to test accuracy in the sense of 15 gallons equals 333 mi. At 22.1 mpg. But the computer keeps a reoccuring track of the gas.
So let's say he started doing that with 200 miles on the car. But never calculated from 0 miles. Wouldn't that be his .7mi difference? Its really a small number. I think if he kept doing it for a few thousand miles. Eventually he will become accuate by .1. That's kinda the point I'm makin.
Feel me?

tbonesteak

Nothing is as accurate as manually calculating your mpg. My maxima is off atleast by 2mpg everytime. It's not that hard to do either.

zilent_jay

How are you filling to the exact same point every time? Let me guess, whenever the pump decides to shut off?

tooohip

No

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davidyan

Actually, I reset the computer each time right when I get back in the car. I think its prob the pump stopping at different points like others have mentioned.