MPG gauge reading too low?

[Vikas]

This used to be dead accurate for 6 years that I have owned the car. For last couple of fills, the tank MPG has been shown almost 10-15% lower. My real calculations have always been done with miles driven and gallons filled. I have not noticed any performance difference in the running of the vehicle.

Any good logical guesses as to what could cause this?

 

[Quattro Pete]

Did you switch to a different tire size?

Still, 10-15% sounds like too big of a difference to be explained by it.

 

[SirTanon]

My thoughts as well - different size tire. Would have to be a pretty notable change in size, too, to get 10-15% change.

 

[shanneba]

Just guessing here with so little info on the vehicle.

Partially clogged fuel injectors? (ECU lengthening injector pulse width/ time but the injector not flowing the calculated gasoline)

 

[Eddie]

Winter gas formulation, 10% to 15% ethanol, Gas station not delivering stated amount. Driving pattern changed. ed

 

[Quattro Pete]

Originally Posted by Eddie
Winter gas formulation, 10% to 15% ethanol,
I can't see how that would impact the car's MPG gauge.

 

[Snagglefoot]

Just guessing, but most cars are using the frequency the pintle within the fuel injector is oscillating at. It's not a flow meter or something like that. The number of oscillations x the amount of fuel that is delivered per oscillation (which was measured in a lab), x the number of cylinders gives the total amount of fuel consumption calculated by the computer. However if the fuel injector is worn (is that possible?) more fuel is getting by per each oscillation. The computer thinks it's getting good mileage but the truth is, it's consuming more fuel than that, as measured by the tank. Like I said, just guessing.

.

 

[KGMtech]

brake pads dragging?

 

[Rand]

so you are getting better mpg than indicated by computer?

Quote

MPG gauge reading too low?

 

[tcp71]

Reading too low compared to what? Hand calculation or previously displayed averages?

 

[Vikas]

Originally Posted by Rand
so you are getting better mpg than indicated by computer?

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MPG gauge reading too low?

Indeed! That is why I am having hard time finding a logical reason. The actual gas mileage of the car as measured by miles driven over gallons filled has not dropped but the car
computer thinks it has. Clogged injector or worn out engine etc will not lead to this specific behavior.

 

[Quattro Pete]

Was the car serviced recently at a dealer? Did they update any software that may have screwed up the MPG computer calcs?

 

[oil_film_movies]

Originally Posted by Snagglefoot
Just guessing, but most cars are using the frequency the pintle within the fuel injector is oscillating at..
Originally Posted by shanneba
Partially clogged fuel injectors? (ECU lengthening injector pulse width/ time but the injector not flowing the calculated gasoline)
Ditto my thoughts. A partially clogged fuel injector (or several) would get a trim addition from the closed-loop O2 sensor feedback control system. So with clogginess (yes, its a word...) it could be commanding a higher flow than normal, yet actually getting normal flow. The engine computer software just uses the commanded flow to calculate MPG.

Run some PEA-based fuel injector cleaner through it, and/or always use TopTier gasoline. Techron cleaner (walmart, auto parts stores), Redline makes a good one too I hear.

 

[shanneba]

Originally Posted by Vikas


Indeed! That is why I am having hard time finding a logical reason. The actual gas mileage of the car as measured by miles driven over gallons filled has not dropped but the car
computer thinks it has. Clogged injector or worn out engine etc will not lead to this specific behavior.


A partially clogged injector would require the ECU to compensate with a longer injector open time to compensate for the reduced flow rate.

The ECU would then use the longer open time (and flow rate of the injector constant) to calculate the amount of fuel injected giving a false higher volume of gas injected, reducing the ECUs calculated gas mileage.

Lower fuel pressure would also cause an error in the ECU calculated mileage for the same reason.

 

[Vikas]

The car was NOT serviced recently. I should check the fuel trims to see if the long term trims are drifting. Shanneba seems to be on right track. Partially clogged injector or lower fuel pressure would cause the error in the ECU calculated mileage but I presume that would get reflected on the fuel trim numbers.