Why your trip computer isn't accurate

[dalesky]

This may belong in the topic

EPA test is totally unrealistic

but I just wanted to get it posted. Any corrections of locations would be appreciated. I apologize for the length. Here is a link to the article-- http://blog.caranddriver.com/why-your-trip-computer-isnt-giving-accurate-mpg-readings-and-how-to-fix-it/

 

 

Here's the article about why it is so difficult to get accurate results of mileage:

 

From Car and Driver Sept. 2013

In our April 2013 issue, we took the EPA to task for its unrealistic fuel-economy assessments of hybrids. But is it possible that the EPA isn’t the only one prevaricating about fuel economy? This is a subject near and dear to your author. Every time I fill up my 2007 BMW 335i, I record the gallons used, the odometer reading, and the trip computer’s mpg report. Given the vagaries of filling a gas tank to exactly the same level at each fill-up, some variability is understandable. But, in years of doing this, I’ve found the BMW’s trip computer has always read higher than the mpg calculations based on the fuel used and the corrected odometer readings. Over the course of 20,340 miles and 56 fill-ups, the mpg inflation ranged from 0.8 to 2.6 mpg. [While Car and Driver’s long-standing policy is to report mileage to the nearest whole number due to our lack of confidence in higher-precision figures, we’re making an exception for this study. Bear with us.] On average, the BMW’s trip computer reported mileage 1.4 mpg higher—nearly 6 percent better—than calculated.

It wasn’t just my car. During a 1289-mile round trip from Ann Arbor to New York, C/D’s long-term Acura ILX averaged 29.5 mpg. But its trip computer claimed 31.4—optimistic by more than 6 percent. We also took the long-term Kia Sportage to Washington, D.C., and back. Over 1064 miles, the Sportage achieved 24.6 mpg according to pump and odometer readings. However, its trip computer padded that figure by 1.1 mpg to 25.7—a 4.5-percent inflation. Every fill-up was slightly optimistic.

Mark Allen, GM’s director of global energy, mass, and aerodynamics, says some inaccuracy is unavoidable. “The density of fuel varies. We have no way to measure it. Mobil might be different than Shell. Summer gas to winter gas could be a big difference.” And then there are vapor-recovery systems. With modern cars’ sealed fuel tanks, gasoline vapor accumulated in the tank is absorbed by a charcoal-filled canister. Periodically, this canister is purged by the engine. “If the weather is hot, you generate lots of purge,” says Allen. “This unmetered fuel isn’t counted by the trip computer.”

Honda uses additional inputs to calculate trip mpg. “We look at the fuel-consumed data that comes from the engine-control computer, but we also track the float sensor measuring the fuel level in the tank,” says Raj Manakkal, chief engineer for electrical and infotainment devices. He also points out that, due to temperature changes, plastic fuel tanks can expand and contract by as much as a liter. On the Acura ILX, that yields a total variation of 4 percent.

These sources of potential imprecision are real, but if you want to accurately track fuel consumption with your trip computer, you need to calibrate it [see below]. On the 335i, a service menu is accessible via an arcane procedure I found on the internet. Within that menu, I was able to change the trip-computer calibration. Changing the default value from 1000 to 1035 aligned the trip computer with reality. Sadly, no such adjustment exists to fix the BMW’s appallingly optimistic speedometer.

CALIBRATION STATION

On your next long trip, stop next to a mile marker and reset your trip odometer. One hundred or more miles later, stop once again and note how your odometer compares to the road mileage. Then, at your next fill-up, reset your trip computer and fill your tank according to this procedure:

1) Fill at the maximum flow rate until the pump clicks off.

2) Wait 10 seconds, then continue filling at the minimum flow rate until the nozzle clicks off again.

3) Repeat this procedure two more times. Ideally, you should do this using the same pump at the same station. Even with a calibrated odometer, a single fill-up has too many variables for an accurate calculation. But if you log at least three fills like this, you should have sufficient data to divide your corrected-odometer miles by the indicated pump gallons to obtain an accurate mpg. Compare that mpg figure to what the trip computer reports and you’ll have your correction factor.

 

 

[acdii]

This is a good post. Keeping it right where it is too!

[dalesky]

This is a good post. Keeping it right where it is too!

I try to not become obsessed with getting the absolute, utmost, whatever it takes, mileage from my Fusion.

On my trip to and from Florida recently I was getting 44 MPG at 65 MPH and 39 MPG at 72 MPH.

In heavy stop and go, high heat driving locally I was getting less than that sometimes.

The total average mileage using the cars computer was 41.5.

Two weeks, many miles, driving as normally as possible.

That to me is hybrid territory, especially given the size, weight, comfort, road handling and load capacity.

I saw a lot of out of state plates on "P" hybrids, and not once did I think that my Fusion was not more desirable, or much more attractive.

Mileage is just part of the equation in my satisfaction quotient.

I am not making a comment here about anyone else, just saying that for me, I am highly satisfied, and glad I bought this car.

I also love this forum, and am glad it is so open, polite and informative.

[acdii]

I try to not become obsessed with getting the absolute, utmost, whatever it takes, mileage from my Fusion.

On my trip to and from Florida recently I was getting 44 MPG at 65 MPH and 39 MPG at 72 MPH.

In heavy stop and go, high heat driving locally I was getting less than that sometimes.

The total average mileage using the cars computer was 41.5.

Two weeks, many miles, driving as normally as possible.

That to me is hybrid territory, especially given the size, weight, comfort, road handling and load capacity.

I saw a lot of out of state plates on "P" hybrids, and not once did I think that my Fusion was not more desirable, or much more attractive.

Mileage is just part of the equation in my satisfaction quotient.

I am not making a comment here about anyone else, just saying that for me, I am highly satisfied, and glad I bought this car.

I also love this forum, and am glad it is so open, polite and informative.

When I got the 13 FFH, I was expecting, based on the current 10 FFH I had, to be getting anywhere from 38-43 MPG in the new one. Well the first one pretty much everyone here knows how well that went, but I can say with pride that the HyTi is doing EXACTLY what I expected out of it. Even when I dont play the "game" I still get 43 or so out of it(like this morning).

 

As long as the car is within 5 MPG of its rating, I am happy with it. The 2010 was always in that range, and this new one show good promise as doing the same.

[hybridbear]

Great info, glad it's pinned. I found with our Fusion that when I adjust the miles for the inaccurate odometer then the calculated fuel use based on gallons at the pump is almost exactly what the dash reads. Based on that knowledge I just accept what the dash says as being close enough to "pass in the dark with a push" as my dad would always say lol

Edited September 24, 2013 by hybridbear

[djminfll]

I usually don't have much of a discrepancy between dash calculation and actual mpg, but my fill-up today was WAY off. I drove 484 miles, and pumped 11.3 gallons, achieving 42.8 mpg. The dash computer told me I had used 10.6 gallons, and got 45.3 mpg. I was scoring 44-50mpg on almost every trip, so 45.3mpg seemed logical. And to make matters worse, I had already driven past 0 miles to empty, so accuracy was really off. What's going on?

[timf]

I just filled my car up for the first time and was shocked by the results. The trip computer had reported 9.88 gallons of fuel used and I was down to just a sliver of yellow on the fuel gauge. I didn't pay much attention to the gas pump while filling, but when it clicked off I looked at the screen and it reported exactly 12.000 gallons. My normal procedure for tracking fuel economy would be to slowly fill to the next click, but since I had already exceeded the trip computer by over 2 gallons and theoretically only had room for about another half gallon of gas given the tank size I just left it there.

 

The net result is while the trip computer told me I was getting 35.8 MPG, the calculations only came out to 29.5 MPG. I have never experienced such a discrepancy in any of my previous cars. The only thing that comes to mind is that I have been using the remote start to pre-warm the engine so I can enter EV mode sooner (and since it has been very cold out). I could see this potentially burning through 2 gallons of gas if every time it used 1/10 a gallon that wasn't picked up by the trip computer. I might try running this tank without any remote starts, then go back to it next time and see what the difference is.

[Waldo]

The first fill up is never accurate, you need to let the car learn the long-term averages before it will be accurate.

[hybridbear]

I just filled my car up for the first time and was shocked by the results. The trip computer had reported 9.88 gallons of fuel used and I was down to just a sliver of yellow on the fuel gauge. I didn't pay much attention to the gas pump while filling, but when it clicked off I looked at the screen and it reported exactly 12.000 gallons. My normal procedure for tracking fuel economy would be to slowly fill to the next click, but since I had already exceeded the trip computer by over 2 gallons and theoretically only had room for about another half gallon of gas given the tank size I just left it there.

 

The net result is while the trip computer told me I was getting 35.8 MPG, the calculations only came out to 29.5 MPG. I have never experienced such a discrepancy in any of my previous cars. The only thing that comes to mind is that I have been using the remote start to pre-warm the engine so I can enter EV mode sooner (and since it has been very cold out). I could see this potentially burning through 2 gallons of gas if every time it used 1/10 a gallon that wasn't picked up by the trip computer. I might try running this tank without any remote starts, then go back to it next time and see what the difference is.

Your car also likely wasn't actually full when you picked it up at the dealer. I reset the trip computer when leaving the dealer and our first tank was off by almost 2 gallons. The car said I'd used 9 and the tank took 11 for example. This is because the dealer hadn't actually filled the tank all the way. As is discussed elsewhere, the first click off usually comes when you can add at least 1 more gallon to the tank. The dealer is likely stopping at the first click off when they're filling the car for you before selling it. They also tend to leave them running idling the ICE and burning fuel.

 

Remote start fuel isn't counted by the trip computer.

Edited April 4, 2014 by hybridbear

[timf]

I suspect you are correct, and the car did not leave the dealership with a full tank despite the gauge being full. I did reset the trip computer when leaving the dealer but in hindsight I should have also topped it off at a gas station to ensure an accurate count. Even if it was only 29.5 MPG that's still 50% more than I was getting before so I'm not complaining.

 

It's also good to know the remote start fuel usage is counted. I assume the timer is counting as well, since it registered about an hour more than I use to represent 100% city driving. Now I'll just have to guesstimate how often I drove on open highway.

[hybridbear]

We've found that the white FFH is also less accurate compared to Fuelly compared to the black one. Long term the black one was off by .1 MPG or about 0.2%. The white one is off by .6 MPG or 1.5%. Either way they've both been really close, but it's annoying that the white car is less accurate on fuel consumption.

[ptjones]

hybridbear did you ever check your odometer with a portable GPS? :)

 

Paul

[corncobs]

Good point it might just be the difference between 17" and 18" wheels.

[hybridbear]

hybridbear did you ever check your odometer with a portable GPS? :)

 

Paul

I have and I adjust my mileage every tank for the discrepancy. The results I listed are after increasing my miles traveled to account for the odometer discrepancy.

[Ted Swing]

I have and I adjust my mileage every tank for the discrepancy. The results I listed are after increasing my miles traveled to account for the odometer discrepancy.

So does that mean the car's odometer tends to underestimate miles travelled?

[hybridbear]

So does that mean the car's odometer tends to underestimate miles travelled?

Correct. My car is off by .1 miles every 5, i.e. the car shows 4.9 miles when I've traveled 5. This translates to the odometer only showing 98 miles for every 100 driven. This is noticeable even on short trips. Once very common trip we make always shows as 6.3 miles in the FFH and 6.4 in the Prius. Another trip always shows as 12.6 miles in the FFH but 12.8 in the Prius. Another that always shows as 11.6 miles in the FFH shows as 11.8 in the Prius. Thus at every fill up I add 2 miles for each 100 miles on the odometer to my miles for that tank.

Edited April 5, 2014 by hybridbear

[Ted Swing]

Correct. My car is off by .1 miles every 5, i.e. the car shows 4.9 miles when I've traveled 5. This translates to the odometer only showing 98 miles for every 100 driven. This is noticeable even on short trips. Once very common trip we make always shows as 6.3 miles in the FFH and 6.4 in the Prius. Another trip always shows as 12.6 miles in the FFH but 12.8 in the Prius. Another that always shows as 11.6 miles in the FFH shows as 11.8 in the Prius. Thus at every fill up I add 2 miles for each 100 miles on the odometer to my miles for that tank.

Interesting. Is this true of all FFHs? Have you ever tested it in another one?

[hybridbear]

Interesting. Is this true of all FFHs? Have you ever tested it in another one?

I don't know. I tested this in the black one on our two road trips. We ran across various "odometer testing" areas where I confirmed this, both in Canada testing kilometers & in the USA testing in miles.

 

The white FFH & black FFH read the exact same miles on the repeat trips mentioned above as well as on longer repeat trips of 50 plus miles. This leads me to believe that the white FFH has the same odometer discrepancy.

[Nick Golden]

Most gps's measure planar miles. Ever wonder why the speed on you gps is off when you go up a hill. Your speedo is measuring the speed of the wheel turning. The gps is measuring your progress across a flat map. They should be pretty close on straightaways.

[rjent]

In theory? or do you have documentation that says that. I ask because I have been unable to find absolute documentation of that or verification of that from the dealership. I think this is a huge issue and I would love to find some definitive info ..... :)

 

Remote start fuel isn't counted by the trip computer.

Edited April 5, 2014 by rjent

[lolder]

The Gen 1 FFH computed mileage is 4% too high but the odometer is 2% too low like in the Gen 2 for a net of 2% too high or about 1 mpg. That's pretty close but since they're all the same you know they could make it exact if they were reguired to. The Prii are much worse.

[ptjones]

The Gen 1 FFH computed mileage is 4% too high but the odometer is 2% too low like in the Gen 2 for a net of 2% too high or about 1 mpg. That's pretty close but since they're all the same you know they could make it exact if they were reguired to. The Prii are much worse.

How much off is your Gen2 FFH odometer? :)

 

Paul

[hybridbear]

In theory? or do you have documentation that says that. I ask because I have been unable to find absolute documentation of that or verification of that from the dealership. I think this is a huge issue and I would love to find some definitive info ..... :)

See this thread http://fordfusionhybridforum.com/topic/8111-fuel-gauge-discrepancies/

[hybridbear]

The Prii are much worse.

You're correct about that. The Prius reads 5-7% too high. Our Prius Fuelly lifetime is just over 3 MPG lower than the Toyota displayed lifetime in the car.

[acdii]

In theory? or do you have documentation that says that. I ask because I have been unable to find absolute documentation of that or verification of that from the dealership. I think this is a huge issue and I would love to find some definitive info ..... :)

This one was easy to prove out. The trip summary when shutting down and starting up after 10 minutes of remote start are identical. I also went 1 week without remote start and one week with it and found an almost 2 gallon difference.