I could use some help with an experiment...

[///M Compact]

My latest:
Trip Computer: 26.1
Actual: 24.03

 

[camrycev6]

My latest:
Trip Computer: 26.1
Actual: 24.03

Excellent. I am going to run at least a couple more tanks without resetting the trip computer to see if it gets more accurate, and of course, by what amount. If by 900 miles it is still 2 or more miles off, I would pretty much consider it worthless.

 

[MikeHTally]

Here are my first results:
174.7 miles, 8.729 gallons = 20.01 mpg
By the computer: 22.1mpg. The computer's error is 10.4% optimistic, 'bout what I've heard. This was a pretty representative sample of a "normal" cycle. That is, some commuting, some playing, some short interstate trips.

 

[imac]

Excellent. I am going to run at least a couple more tanks without resetting the trip computer to see if it gets more accurate, and of course, by what amount. If by 900 miles it is still 2 or more miles off, I would pretty much consider it worthless.

i wouldnt say it'd be worthless... if it's consistantly off my 2-3 mpg then why not just take the reading from the dash and chop the number down by 2.5. unless people need to have extremely accurate readings for whatever reason, i see no problem with going by what the dash reading says as long as i realize that it's going to be a couple mpg optimistic.

 

[MikeHTally]

...problem with going by what the dash reading says as long as i realize that it's going to be a couple mpg optimistic.

Much like speedometer error... Just be aware and live with it. One thing I forgot to do on my first test is compare the odo with my GPS unit. I have compared the speedometer and it's VERY close -- less than 1/2 mph.

 

[ericrapp]

Mine at 147 was dead on. My dear friend doubted my ability to achieve the speeds I claimed I had experienced. He and the other observer agreed that the 147speedo was 147 GPS and they put their seatbelts back on. so imo the speedo on stock wheels and tires were quite correct. down to computed 28.7mpg

 

[tiagotiago]

First tank.

Calculated Km/L : 7.456 (converted to mpg: 17.614)
Computer L/100Km: 11.4 (converted to mpg: 20.723)

Notes:
-Computer's Liters/100 Km as displayed on the car's trip computer. Was reset on the last gas fill-up.
-Filled up in the same gas station, at the same pump.
-Milage was in 95% city driving 5%highway.
-1 Mile = 1.6km 1 Gal = 3.78 L

 

[m5rMS3]

I dont remember actual numbers, but I do manually calculate the MPG each fill up and compare it to the computer reading which I also reset each fill up. On average I would say that the computer reads about 2-4 MPG higher than actual. However, it seems like the higher my mileage, the further off the computer estimate is. The computer has read as high as 29MPG, but when it is that high the actual number is more like 25 or 26. When I do a lot of city and fun driving and the computer reads something like 24 MPG the actual will be very close...maybe 23 MPG. Hope this helps.

 

[///M Compact]

I still wonder if there's a way to adjust the mpg calculation like you can on BMWs.

 

[Bluedot]

I make frequent trips from my house to my dads place, which is exactly 326 miles round trip, door to door. All freeway miles (maybe 5 miles total of city driving)

Every time I go, I fill up at the same station between my house and the freeway, reset trip-odo, and trip computer. Ive played with the computer quite a bit and its typically 10% off of actual.

Last trip was 12.3 gallons used (26.5 mpg) and computer reported 29.7

 

[m5rMS3]

I still wonder if there's a way to adjust the mpg calculation like you can on BMWs.

How do you do that. I just came from an e46 and i never learned about that feature. Although I just found the BMW flashlight in the glove box about a month before i traded it in.

 

[ericrapp]

I make frequent trips from my house to my dads place, which is exactly 326 miles round trip, door to door. All freeway miles (maybe 5 miles total of city driving)

Every time I go, I fill up at the same station between my house and the freeway, reset trip-odo, and trip computer. Ive played with the computer quite a bit and its typically 10% off of actual.

Last trip was 12.3 gallons used (26.5 mpg) and computer reported 29.7

Iguess I will actually have to fill the car up a couple times and compare. that # seemed mighty high for my driving style.

 

[///M Compact]

Today's fill-up:
Trip Computer: 25.6
Actual: 24.31

 

[camrycev6]

Ok... I am on my third tank, and it doesn't look good for the trip computer.

Here are the current results from the second tank. (625 miles total.)

Running average (actual) = 23.9 MPG
Running average (computer) = 26.5 MPG.

The system is still off by 2.6 MPG. That is pretty bad for 625 travelled miles. After the third tank, if the computer doesn't improve, I think we can pretty much conclude it is a useless tool for evaluating MPG. I have read the posts and the vast majority is off by 2 or more MPG.

 

[MikeHTally]

My results from the second element:
Actual: 204.2 miles/10.003 gals=20.41 mpg Yeah, I go "play" some.
The computer: 22.7 mpg 11.2% optimistic.
So, the computer is still 2 mpg +/- high.

 

[camrycev6]

My results from the second element:
Actual: 204.2 miles/10.003 gals=20.41 mpg Yeah, I go "play" some.
The computer: 22.7 mpg 11.2% optimistic.
So, the computer is still 2 mpg +/- high.

Bingo Mike. That is exactly what I experience. It always seems to be high by 2+ MPG. The one hypothesis that was offered in this post states that as you drive more (without resetting the computer) it gets more accurate. I have not really found that to be the case, but I figure after 900+ miles, I can make a solid conclusion. I only have about 180 to go!

 

[tiagotiago]

Second tank:

Calculated Km/L : 6.961 (converted to mpg: 16.445)
Computer L/100Km: 12.2 (converted to mpg: 19.365)

Notes:
-Computer's Liters/100 Km as displayed on the car's trip computer. Was reset on the last gas fill-up.
-Filled up in the same gas station, at the same pump.
-Milage was all city driving
-1 Mile = 1.6km 1 Gal = 3.78 L

 

[camrycev6]

Second tank:

Calculated Km/L : 6.961 (converted to mpg: 16.445)
Computer L/100Km: 12.2 (converted to mpg: 19.365)

Notes:
-Computer's Liters/100 Km as displayed on the car's trip computer. Was reset on the last gas fill-up.
-Filled up in the same gas station, at the same pump.
-Milage was all city driving
-1 Mile = 1.6km 1 Gal = 3.78 L

Thanks Tiagotiago. Again, we are seeing similar results. I only have a 1/4 tank left on my third tank. I think we will be able to put this to rest after that. If the computer can't even get close after 800+ miles, it is essentially worthless to me.

 

[camrycev6]

Ok... Final data... and a conclusion.

922 total miles without resetting the computer.

38.25 gallons = 24.1 miles per gallon.
Computer (once again) reported 26.5 MPG

Difference of 2.4 MPG. Last tank the running average was 2.6 MPG.

That is an error of (2.4 x 38.25) 91.8 miles! (Or 9.95%) Frankly, that is horrible.

Conclusion: The trip computer for determining MPG is highly inaccurate at all ranges of mileage, regardless if you reset it or not. It does not increase significantly in accuracy over time.

Thanks to everyone who helped me out with this.

 

[MikeHTally]

Interesting. I just did my last calculation. The computer's "optimism" isn't quite what it was... Last actual mileage was 20.61 and the computer thought it was 22.2, only a 7.7% difference. Last "fill it to the brim", though. From now on, it's pump cutoff rounded to the next dollar amount.