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Discussion Starter · #1 ·
So far I have five fill-ups in my '10....

CALCULATED MPG / TRIP COMPUTER MPG / DEVIATION

18.5 / 18.5 / 0.0
16.2 / 16.7 / 0.5
18.0 / 18.0 / 0.0
16.5 / 16.1 / (0.4)
18.4 / 18.4 / 0.0

For the first 5 fill-ups (I don't count the initial fill-up since I don't trust the dealer to have properly reset the trip odometer) the trip computer has been spot-on 3 times with an overall accuracy of -0.4 / +0.5 (largely due to different fill quantities at different pumps) for an average error of 0.1 MPG high. I'd say that's plenty accurate for a trip computer - hard to ask for much better!
 

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While I find my trip computer MPG somewhat accurate most of the time I can't say mine is anywhere near to what you are seeing on yours. Mine has fluctuated about +/- 1.5 MPG and sometimes almost spot on. I never really trust my computer for actual MPG and always calculate it out when refueling. It's a good reference for me and that is all. It is surprising they are as accurate as they are though.
 

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Mine seems to always be too optimistic - sometime by over 1 mpg.

My Scangage II is too pessimistic and the real MPG is somewhere between the 2 estimated MPG.

Typically, the Ridgeline will say 23 and the Scangage will say 22 and the real calculated MPG would be 22.5.
 

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I've found the Ridgeline's trip computer pretty precise. Now, if you top off, or keep filling past the auto shut off, then of course it'll be off. The trip computer measures exact amount of fuel usage as calculated by the ECU and injectors. It should know the exact amount of fuel used. With the introduction of the MAF sensor in 2009 the computer will be able to calculate air vs fuel a bit more accurate.
 

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I initially found mine to be a bit optimistic. However, more recent road trips show it to pretty much bang on when comparing to fuel/odometer measurements. Could there be an inaccuracy that only shows up before the engine is broken in?
 

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Discussion Starter · #6 · (Edited)
65 fill-ups over 17K miles since new. The blue line is the manual calculation. The orange line is what the trip computer read. I don't believe one could ask for better accuracy. I'm curious if anyone else has a similar graph for comparison. I also log all other available data such as elapsed time, average speed, and remaining oil life at each fill-up, even though I haven't analyzed elapsed time or average speed...yet. As you can see, there are very slight variations (probably due to different pumps, fill angles, etc.), but overall it's virtually dead on.
 

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Nice graph. I think I will start putting in my fuelly notes what the trip meter shows at the time of fill ups. I have noticed that mine isn't more the 3 tenths off either.
 

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No trip computer on my 08; the wife's CR-V has a trip computer and it shows 10% more MPG than the manual calculations. This is consistent!!!
 

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No trip computer on my 08; the wife's CR-V has a trip computer and it shows 10% more MPG than the manual calculations. This is consistent!!!
But also, it's a CR-V, not a Ridgeline. :act002:
 

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Discussion Starter · #10 ·
Something interesting that I just noticed while looking back at this graph... If you added a trend line to this graph, you would see that:

1. Fuel economy went DOWN as the miles accrued since it was new.
2. Fuel economy went UP after the first oil change, then headed back DOWN.
3. It went UP again after the second oil change, and has slowly gotten better and has since leveled off at around 19 MPG as evidenced by several more fill-ups since I posted this graph.
 

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zroger, nicely done! Good info to know. Thanks for sharing. I should start keeping track of mine as well. I just hit the reset button and note the avg mpg.
 

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zroger, nicely done! Good info to know. Thanks for sharing. I should start keeping track of mine as well. I just hit the reset button and note the avg mpg.
Fuelly has been a good help for this, for me.

By itself it's just another Web form to fill out. But if you have a smartphone like an Android or iPhone, the essential details are easy to enter while you do your fuel-up, or right afterwards.

Yes, it's geeky, but it's a way to get your fuel-ups into an analysis program without intermediate steps (like pencil and paper notes). Click the "Fuelly" block in my signature to get there, and see what a fuel history looks like.
 

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Discussion Starter · #13 · (Edited)
As of 11/16/2013, I've logged each of the 163 fill-ups since I purchased the truck new in April 2010. The average of all manually-calculated MPG readings is 17.9. The average of all MPG readings according to the trip computer at the time of fill-up is...17.9. What does this mean? It means that over 45K miles and 3.5-years of ownership, the trip computer has been accurate to better than a tenth of a mile per gallon. One can therefore conclude that the trip computer is more accurate than a manual calculation since the manual calculation will vary depending on fill level. It is practically impossible to fill the vehicle with precisely the exact amount of fuel each and every time. No matter what method I use to compare the manual calculation vs. the trip computer, the result always confirms the same level of accuracy.

More interesting facts: My average speed over the past 45K miles has been 34.6 MPH. The average engine running time between fill-ups is 7 hours, 33 minutes. The average fill-up is 15.452 gallons. I've spent $8,217.18 on gasoline - about $195 per month. The average number of miles until the oil life monitor hits 15% is 7,750.
 

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Wow, amazing data. My '13 RL started out very close to the manual calculations but now is showing .5 to 1.0 less than the trip computer.
 

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And how are you counting the miles? Using the car or GPS.

On the only trip I take notice of MPG (or used to) which is 945 via GPS and 967 via the car. That makes a difference in the calculations.
 

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Discussion Starter · #16 · (Edited)
And how are you counting the miles? Using the car or GPS.

On the only trip I take notice of MPG (or used to) which is 945 via GPS and 967 via the car. That makes a difference in the calculations.
Good point. Your claim represents a 2.3% error or less than a half a mile per gallon at 20 miles per gallon - still more than suitable for government work! :)

Here is a graph showing the deviation between my manual calculation and the trip computer. Aside from a blips on the 95th and 96th fill-ups (which were likely caused by an "improper" fill, the deviation is generally less than half a mile per gallon at each fill, but dead-on over time.
 

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I don't know how a trip computer can be any more accurate than manual calculations.

I log everything on Fuelly and also on an Excel spreadsheet. ;)

The largest peak on my RL chart is during the time I used ethanol free 87 octane gas (Fall/Winter 2011-2012). I filled it up consistently (at that time I was filling to just short of the brim, so I know the fillups were consistent... same pump, same spot). The trends seem easier to spot when I look at the rolling 10 fillup average (blue dots on the RL and pink dots on the Pilot). The graph periods cover the same period of time.

RL chart:


Pilot chart, for comparision:


One thing I've noticed over the years is that using the tripmeter on the odometer to record my miles traveled between fuelups (to the 1/10th) and calculating my odometer miles, my ACTUAL odometer reads 7 miles higher than the calculated miles (adding up all the trip miles). On the Pilot (as it approaches 100k miles, the odometer reads 9 miles higher than the calculated miles (adding up all the trip meter miles). I guess that's just an accumulation of error over 192 fillups (RL) and 340 fillups (Pilot) in the tripmeter readings. But I would think that the over/under 1/10th readings would balance out over time. [shrug]

For instance, at my last fillup, my calculated odometer reading (adding all the individual tripmeter readings was: 62,583 miles (62,582.5 trip meter miles) while the odometer actually read 62,590 miles.

The Pilot calculated: 99,065 (99,064.8 trip meter) while the odometer reading is actually showing 99,074 miles.
 

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And how are you counting the miles? Using the car or GPS.

On the only trip I take notice of MPG (or used to) which is 945 via GPS and 967 via the car. That makes a difference in the calculations.
Using Fuelly compared to trip computer display. Same on the wife's CR-V. CR-V started out very accurate now off as much as 1 mpg.
 

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Discussion Starter · #19 ·
I don't know how a trip computer can be any more accurate than manual calculations.
Clarification: I didn't intend to imply that I felt or concluded the trip computer was "more accurate" overall. My intended meaning was that it appears (at least in my case) that the trip computer gives a more meaningful indication of fuel economy at each individual fill. For example, if there is a 0.5 MPG discrepancy between the manual calculation and the trip computer, the most reasonable assumption is that the manual calculation is off due to a difference in fill amount. With rare exception, I've used the same pump at the same gas station for the last 3.5 years. Still, there is a small discrepancy each time - probably due to:

1. Different fuel temperatures. Granted, the tanks are underground where the temperature is relatively consistent, but the fuel comes in on trucks. In the winter, the fuel is cool. In the summer, the fuel is hot. Depending on the frequency at which the station receives fuel and other variables, it is very likely the temperature and volume of the delivered fuel varies.

2. The mechanical hysteresis/inconsistency/slop of the fuel pump nozzle's auto-shutoff.

3. The fluid dynamics (flow and foaming characteristics) of the gasoline pouring into the vehicle's filler neck and the resulting effect on the point at which the pump will automatically shut off.

For example, we might be using the same pump at the same station, but we don't know if they've changed nozzles (because someone drove off with the previous one) with a different shutoff point (an ounce or two can make a difference in your manual calculation) or perhaps they've changed the actual pump resulting in a different flow rate (which can affect how much the fuel foams or "backs up" into the filler neck). Even the angle of the filler handle can alter the auto-shutoff point.

Now, here's something else to consider that'll blow your skirt up... The trip computer knows how much fuel the vehicle has used based on injector pulse width. Given the injectors flow a specific amount of fuel in a given amount of time at a given amount of fuel pressure, the amount of fuel dispensed can be calculated. This assumes the injectors and fuel pressure are operating perfectly within specification. Since the injector timing is controlled by the ECU based on the air:fuel ratio as determined by the oxygen sensor in closed-loop operation (where the engine spends most of its time), the accuracy of the trip computer is greatly influenced by the accuracy and condition of the oxygen sensor. If the sensor becomes skewed, so will the accuracy of the trip computer. Another factor is tire wear. As the tires wear or are under-inflated, they will rotate slightly faster which will cause the odometer to read slightly higher resulting in slightly better fuel economy readings than exist in reality.

These are nit-picky details that most people don't (and probably shouldn't) think about in daily life else they might end up in an insane asylum. Nonetheless, they ARE real factors that determine the accuracy of manually calculated fuel economy readings and those indicated by the trip computer.
 

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Clarification: I didn't intend to imply that I felt or concluded the trip computer was "more accurate" overall. My intended meaning was that it appears (at least in my case) that the trip computer gives a more meaningful indication of fuel economy at each individual fill. For example, if there is a 0.5 MPG discrepancy between the manual calculation and the trip computer, the most reasonable assumption is that the manual calculation is off due to a difference in fill amount. With rare exception, I've used the same pump at the same gas station for the last 3.5 years. Still, there is a small discrepancy each time - probably due to:

1. Different fuel temperatures. Granted, the tanks are underground where the temperature is relatively consistent, but the fuel comes in on trucks. In the winter, the fuel is cool. In the summer, the fuel is hot. Depending on the frequency at which the station receives fuel and other variables, it is very likely the temperature and volume of the delivered fuel varies.

2. The mechanical hysteresis/inconsistency/slop of the fuel pump nozzle's auto-shutoff.

3. The fluid dynamics (flow and foaming characteristics) of the gasoline pouring into the vehicle's filler neck and the resulting effect on the point at which the pump will automatically shut off.

For example, we might be using the same pump at the same station, but we don't know if they've changed nozzles (because someone drove off with the previous one) with a different shutoff point (an ounce or two can make a difference in your manual calculation) or perhaps they've changed the actual pump resulting in a different flow rate (which can affect how much the fuel foams or "backs up" into the filler neck). Even the angle of the filler handle can alter the auto-shutoff point.

Now, here's something else to consider that'll blow your skirt up... The trip computer knows how much fuel the vehicle has used based on injector pulse width. Given the injectors flow a specific amount of fuel in a given amount of time at a given amount of fuel pressure, the amount of fuel dispensed can be calculated. This assumes the injectors and fuel pressure are operating perfectly within specification. Since the injector timing is controlled by the ECU based on the air:fuel ratio as determined by the oxygen sensor in closed-loop operation (where the engine spends most of its time), the accuracy of the trip computer is greatly influenced by the accuracy and condition of the oxygen sensor. If the sensor becomes skewed, so will the accuracy of the trip computer. Another factor is tire wear. As the tires wear or are under-inflated, they will rotate slightly faster which will cause the odometer to read slightly higher resulting in slightly better fuel economy readings than exist in reality.

These are nit-picky details that most people don't (and probably shouldn't) think about in daily life else they might end up in an insane asylum. Nonetheless, they ARE real factors that determine the accuracy of manually calculated fuel economy readings and those indicated by the trip computer.
Great post Roger!

Neglected maintenance items I've seen cause MPG to start trending downward on vehicles with over 100,000 miles.

1) O2 sensors. While a weak O2 sensor will not throw a Check Engine Light (CEL) code, losing sensitivity will cause MPG to drop. I try to replace O2 sensors after 100,000 miles and definitely by 130,000.

2) A dirty Mass Air Flow (MAF) sensor and air filter will cause MPG to drop. Most people know to change the air filter. Many people do not know to "carefully" clean the MAF.

MAF cleaning video.

http://www.youtube.com/watch?v=21_tq2ETQZc&list=PL57CEF1B5BF40926B&index=8

O2 sensor video

http://www.youtube.com/watch?v=oSPZ5rl0QH8
 
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