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What have I done?! MPG has tanked after Magnaflow


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I don't know what has happened, but since I installed a Magnaflow muffler and removed the flapper, my gas mileage has tanked. I was hoping the computer just needed time to get adjusted, but that hasn't been the case. I've had it done since last November. Pre-install, I averaged 17.2 to 17.5 mpg mixed driving and could get 20 to 21 on a 90% interstate trip. Post-install, I'm getting at best 15.9 mpg mixed driving and abysmal mpg for interstate travel. For example, I went on a 300 mile all Interstate trip yesterday, averaging my typical speed (75 to 80 mph) and I ended the trip with an average of just 16.1 mpg! Other than trying to find a stock muffler and going back to the minivan sound to get my mpg back, is there anything that can be done? Also, my guess is we are still on winter gas in Tennessee (or are we), but surely that can't be the difference given the large variance?

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I just replaced my Flowmaster with 2 Magnaflows and didn't notice a gas mileage drop at all. Last week I took a 300 mile trip, 80% interstate, driving 75-85 mph and got 18.2 mpg with the air conditioner running on my 6.2.

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Your right foot and your ears got much happier after you installed it... :smoker:

Not at all. For a week or two afterward, yes. But that has one since worn off lol. That wouldn't explain the interstate mpg either.

 

Yes, I've read the myth about backflow pressure.

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I don't know what has happened, but since I installed a Magnaflow muffler and removed the flapper, my gas mileage has tanked. I was hoping the computer just needed time to get adjusted, but that hasn't been the case. I've had it done since last November. Pre-install, I averaged 17.2 to 17.5 mpg mixed driving and could get 20 to 21 on a 90% interstate trip. Post-install, I'm getting at best 15.9 mpg mixed driving and abysmal mpg for interstate travel. For example, I went on a 300 mile all Interstate trip yesterday, averaging my typical speed (75 to 80 mph) and I ended the trip with an average of just 16.1 mpg! Other than trying to find a stock muffler and going back to the minivan sound to get my mpg back, is there anything that can be done? Also, my guess is we are still on winter gas in Tennessee (or are we), but surely that can't be the difference given the large variance?

Winter gas is mostly a myth. It has only very slightly less energy content - 0.5% less maybe. The winter fuel economy drop has more to do with the cold air temps which increase friction, increase engine/tranny warmup times, increase heater demand, increase air density, lower tire air pressure, etc.

 

I'm betting most of your problems stem from enjoying the noise of your new muffler and colder temps.

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Complaining about 16.1 mpg at 80 mph in the hills of TN in a 4 wd truck?

 

Part joking & part serious. Mpg's really seem to nose dive in most anything past 65 mph or so.

 

Have a 2013 6.2L so not the same as your truck. But the winter fuel mileage always seems to be 14.8 w/summer at 16.2, same mixed driving.

That summer number never seems to get here until around mid May or so( & very small if any differences in warm up times etc).

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I drive the hills of TN/VA and after 1.5 years and 11k miles my life time average is 19 mpg. 17 or so in the winter and 21 or so in the summer. Still have stock muffler (did run a dynomax but didn't like it).

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Complaining about 16.1 mpg at 80 mph in the hills of TN in a 4 wd truck?

 

Part joking & part serious. Mpg's really seem to nose dive in most anything past 65 mph or so.

 

Have a 2013 6.2L so not the same as your truck. But the winter fuel mileage always seems to be 14.8 w/summer at 16.2, same mixed driving.

That summer number never seems to get here until around mid May or so( & very small if any differences in warm up times etc).

Again, doesn't explain the difference between pre muffler install and post muffler install mpg under similar conditions (speed, etc). We're talking 4 mpg difference, not .5 or even 1 or 2 mpg difference.

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