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A question related to Car average mpg display correctness I suppose, but say if a piggy back chip (e.g. http://www.dieselandpetroltuning.co.uk/) was added to a diesel car with trip computer, would the trip computer take into account the extra fuel injected via the remap/chip, or would the increase in fuel consumption be ignored and make the trip computer less accurate?

Sites selling chips such as these claim

Improved MPG for high mileage drivers

but to test these claims, would you have to measure the relative consumption manually rather than use the relative values reported by the trip computer before and after fitting?

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Can you provide more application-specific information? My first thought would be that a chip shouldn't interfere with the trip computer's ability to read how long the injectors are open for--which is how the fuel consumption is typically calculated. –  mac Sep 3 '13 at 19:23
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@mac The answer to the linked question suggests that the amount of fuel to be sprayed is calculated, but the trip computer uses the intended amount rather than the actual amount. If the chip causes more fuel to be sprayed, does the trip computer still believe that only the intended amount was sprayed, not the amount overridden by the chip? –  SilverlightFox Sep 4 '13 at 10:34
    
The question I was driving at (and what I believe you'd need more information on the specific chip to determine) is whether the factory ECU sees the actual injector opening times, or whether the "piggyback chip" comes in downstream and asks for more fuel without the factory ECU knowing. Answer this question, and you've answered your question. I've done performance chips on two of my vehicles, (one re-flash, one solder-in), and in those cases, the chip played nice with the factory ECU such that the true, "chipped" operating state was known to the factory ECU. –  mac Sep 4 '13 at 14:56
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2 Answers

Great question. I am making an educated guess here, so don't quote me in court;)

Because piggyback chips interfere with your car's various inputs I would assume that you are correct and that your MPG score will be inaccurate as a result. I can't say by how much though. These chips usually give an MPG boost of around 10%, so I would imagine your true MPG would be 10% less than your trip computer says it is. Not that it was 100% accurate to begin with. Toyota engines have an AIr/Fuel ratio sensor that is much more accurate than the usual Oxygen sensors found in most other engines, so you're probably coming of an inaccurate base to begin with.

If it makes you feel any better, my Opel reports my fuel consumption to be 10% less than it actually is, so if I were in your shoes, it'd probably improve the accuracy;)

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Any reference regarding your statement about how only Toyota engines measure the fuel volume passing through the injectors? This doesn't sound correct to me. –  mac Sep 3 '13 at 19:21
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Sure. I came across this while developing an OBD app for my car. Toyota has an Air/Fuel ratio sensor, while most other cars have an Oxygen sensor: motor.com/article.asp?article_ID=1643. Although it seems I was wrong about the position and the exact function of the sensor. –  Juann Strauss Sep 4 '13 at 8:16
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There's also this PDF which discusses the advantages: autoshop101.com/forms/h37.pdf –  Juann Strauss Sep 4 '13 at 8:19
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Neither an air/fuel ratio sensor nor an oxygen sensor measure fuel consumption. Fuel consumption is measured at the injectors. –  mac Sep 4 '13 at 14:36
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Actually not. It is arrived at by the formula MPG =VSS * 7.718/MAF, as seen here: stackoverflow.com/questions/17170646/… –  Juann Strauss Sep 4 '13 at 16:10
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Like @mac said, it all depends on what the piggyback/chip does. If it modifies the signal to the injectors and the ECU does not see these changes, then your MPG-o-meter will be off.

However, if it is like a "reflash" where the ECU uses new values for it's load vs RPM fuel injection tables, then it should still be as accurate.

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