# Trip computer woefully inaccurate in calculating fuel economy



## CDJackson (Feb 28, 2013)

We just got back from a long road trip involving multiple fill-ups. The trip computer was consistently telling me that I was getting around 27 MPG on each fill-up. I was very excited until I divided miles driven by gallons of gas required to fill up the tank and discovered I was actually getting closer to 24 MPG on each fill-up. What gives??? The van knows PRECISELY how many miles I've driven since I hit the "reset" button on the computer, and knows with at least some accuracy (I assume?) how much fuel has been used to drive those miles. It's a simple calculation. So why is it so far off? To put it in perspective, it took about two gallons more to fill the tank on each fill-up than it should have according to the trip computer. This happened consistently across three separate near-empty-to-completely-full fill-ups.

This is on a 2012 with the Pentastar engine. I'm not unhappy about the fuel economy. It's close to the EPA estimate and we were driving 70-80 mph over roads that had significant elevation changes. I'm just unhappy about being "tricked" into thinking I'm getting better fuel economy than I really am.


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## Artem (Dec 26, 2012)

I think it is all within normal variations. 
First, the gas pumps have some inaccuracy. In Russia they would tweak the pump, so it consistently gives 7% less fuel than it charges for.
Secondly, how much fuel fits in your gas tank varies with vehicle position and angle.
Thirdly, the amount of gas consumed is calculated by multiplying interval when fuel injectors are open by their predefined flow rate. Seems to me that the flow rate can vary quiet a little bit.

I personally don't care much about fuel economy, but during my last 400 mile interstate trip the meter showed me 23.5 MPG.


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## CDJackson (Feb 28, 2013)

I doubt it's the gas pumps. This is not Russia. The pumps are regulated and the stations can incur major fines if they're miscalibrated. I'm sure there is some minor variation from fill-up to fill-up, but this was a full 3 mpg difference at every fill-up. That's a ~12% difference, or about 2 gallons per fill-up, which I think is considerably more significant than a rounding error. Plus, the discrepency ran consistently in one direction. If it was due to random variations you would expect to see errors on both sides of the actual mpg number.

Having said that, three fill-ups is probably too small of a sample size to know for sure. I'll have to keep an eye on it.


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## Zambee500 (Jun 23, 2011)

A couple of thoughts. 

I have an '09 Rout - let's call it a 1st gen - and the on-board computer is fairly accurate. It's pretty consistently off by only about half an MPG more optimistic than pencil/paper calculations. Every time I've compared it to pencil/paper it has always been optimistic by just a hair - never dead-on and never lower than pencil/paper calculations. So I call that "normal". Others here with the '09 and '10 have reported the same, but not sure I've seen anyone with the 2nd gen '11/'12 chime in with their experience, so that could be a difference.

From what I've gathered on the TDI forums, the on-board computers on the early-to-mid 2000s VWs were always about 10% off too optimistic compared to pencil/paper. My '05 Passat TDI has always been 3-4 MPGs higher than the pencil/paper calculations. So even though it's considerably off, it's what I also call "normal" based on what everyone else seems to have. Similarly, the speedometer has always been 3-4 MPH off too, comparing it to both my GPS/navi and also every time I've driven past one of those DOT signs with the radar that displays how fast you're driving. I also consider it "normal" compared to what others have reported who drive the same generation VWs. So there's some bias built into the computer system. So what does VW's history have to do with Chrysler-built Routans? Nothing really, except...

If other 2nd gen Routs (the '11 and '12 model years w/ Pentastar engine) are similar to yours and the computer display is always off (optimistic) by 10-12% or 3-4 MPGs, then there's probably nothing that can really be done to change the computer programming and it's just "normal". If others aren't reporting the same thing, however, then go to the dealership and have them check it out while it's under warranty. And keep in mind if the computer is miscalculating the MPGs then your speedometer is also likely displaying inaccurate speeds. Worth having the dealership check that out too. Or at least be aware of it when Johnny Law is around.


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## blazeoc (Feb 1, 2013)

That happens to me every time it seems. It will tell me 21 miles per gallon and in reality it is close to 19.6 or so. 

I am always about a mile and a half to 2 miles off.


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## CDJackson (Feb 28, 2013)

I've been watching over the last couple months and I'm certain now that the trip computer is just plain wrong at calculating fuel economy. It ALWAYS significantly overestimates our actual mileage. Our last fill-up, the trip computer said 27.2 mpg. Actual mileage (miles driven divided by gallons used) was 24.7 mpg.

If I were a conspiracy theorist I'd say the manufacturer wants to convince owners they are getting better fuel economy from the van than they actually are.


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## blazeoc (Feb 1, 2013)

CDJackson said:


> I've been watching over the last couple months and I'm certain now that the trip computer is just plain wrong at calculating fuel economy. It ALWAYS significantly overestimates our actual mileage. Our last fill-up, the trip computer said 27.2 mpg. Actual mileage (miles driven divided by gallons used) was 24.7 mpg.
> 
> If I were a conspiracy theorist I'd say the manufacturer wants to convince owners they are getting better fuel economy from the van than they actually are.


Without a doubt. Mine is like I said a minimum of 1 1/2 miles off with it at times three. 

Sent from my SCH-I545 using Tapatalk 2


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## Steveaut (Sep 16, 2010)

Personally, I have yet to see any vehicle's computer come up with the same number as hand calculating. The computer on all of the vehicles I have seen always show higher MPG than the math done at the pump. My Jetta TDI is always off as was my Durango, Journey and Ram. My friends' Cherokee, Silverado (Duramax), Wrangler, Beetle and Sonata are always off as well. In fact my brother-in-law used to brag up what his Duramax Silverado was getting on the computer until I made him hand calculate at the pump. My father-in-law did the same thing. I would suspect that an industry standard has been inadvertently developed (software code written) in which the computer's calculations err on the side of higher MPG. I suppose to be competitive in that area the brands all have to play the same game. I wouldn't sweat it. All I can tell you is that getting 24 MPG on your van is a homerun. Be happy about that. I only get about 19-21 (4.0 motor).


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## CDJackson (Feb 28, 2013)

Yeah, I'm not at all disappointed by the mileage--just the accuracy of the computer. Frankly, I'm amazed that I can haul 2.25 tons of van + another 700 lbs. or so of people and stuff down the freeway with frequent traffic back-ups and elevation changes and still get close to 25 mpg. That's significantly better than my little 4-cylindar Subaru gets with just me in it!


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