MPG Gauge Question
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MPG Gauge Question
Ok just installed a straight pipe and now my mpg electronic gauge is going is doing some weird crap. The Exhaust Has helped my mpg, but Now anytime I stop driving, (at a stop light) or I have it in park and still running, the mpg gauge just starts dropping, from 18ish to 4ish. If I reset it, It usually goes to about 9, but it seems if the engine is really warmed up and if I then reset it, the gauge stays normal. Why is the gauge dropping at idle? Doesn't make any sense to me at all. Thanks!
Frmr Ford Trans Engr
Joined: Aug 1999
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From: SE Florida
Because you are getting zero MPG at idle. The equation is miles divided by gallons. At idle the miles traveled is zero, and the engine is using fuel. Zero miles divided by any amount of fuel is zero.
To take it to the extreme you could fill the tank and leave the truck idling at the pump until it ran out of fuel. You would have driven zero miles and used an entire tank of fuel. Zero MPG.
To take it to the extreme you could fill the tank and leave the truck idling at the pump until it ran out of fuel. You would have driven zero miles and used an entire tank of fuel. Zero MPG.
Senior User
Joined: Apr 2012
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From: Orlando, FL
my guage also gives me 9999 miles per gallon 
Normally when I completely let off the gas at 70 mph amd let it coast.... for the same reasons... If you have a scan guage for electronic guage there should be another indicator of gallons per hour.... it is a more accurate reading of fuel consumption since it only has one variable (fuel demand) while miles per gallon has 2 (fuel consumption and speed overland).
Normally when I completely let off the gas at 70 mph amd let it coast.... for the same reasons... If you have a scan guage for electronic guage there should be another indicator of gallons per hour.... it is a more accurate reading of fuel consumption since it only has one variable (fuel demand) while miles per gallon has 2 (fuel consumption and speed overland).
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Junior User
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Because you are getting zero MPG at idle. The equation is miles divided by gallons. At idle the miles traveled is zero, and the engine is using fuel. Zero miles divided by any amount of fuel is zero.
To take it to the extreme you could fill the tank and leave the truck idling at the pump until it ran out of fuel. You would have driven zero miles and used an entire tank of fuel. Zero MPG.
To take it to the extreme you could fill the tank and leave the truck idling at the pump until it ran out of fuel. You would have driven zero miles and used an entire tank of fuel. Zero MPG.
Posting Guru
Joined: Jun 2012
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My Scangauge has two stock options for fuel mileage. One is the average for that trip, and the other is instantaneous. The average doesn't change at idle unless it's a really short trip, the instantaneous drops to zero in a second or so.
Average is named AVG, instant is named MPG, I think.
Brian
Average is named AVG, instant is named MPG, I think.
Brian
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Junior User
Joined: May 2010
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My Scangauge has two stock options for fuel mileage. One is the average for that trip, and the other is instantaneous. The average doesn't change at idle unless it's a really short trip, the instantaneous drops to zero in a second or so.
Average is named AVG, instant is named MPG, I think.
Brian
Average is named AVG, instant is named MPG, I think.
Brian
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