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Discussion Starter #1
Hello. I got a great deal on a Spintech muffler. It is 2.5”/dual in a s 3” single out. If you are familiar with the muffler, it has 8 “coils” in it. I love to Do a few full throttle on ramp runs a few times a week. I also will do some dorky hyper-mileage-ing on fwy trips. My mpg’s are lower than they have ever been. With just a Hypertech, I have got 17-19 on some fwy trips (factory 255 tires). Now it is low 14’s.

Initially I was flooring it a lil’ more often to near the noise, but not anymore.

I disconnected the battery for a reset too. Still the same.

Has this happened to others?
 

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Engines need back pressure to operate properly as your example clearly demonstrates. Still surprised at how many people think that the engineers at the various automotive manufacturers are stupid and some guy operating out of a small shop can provide a part that will dramatically improve performance in some way. All they do is lighten your wallet.
 

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Hello. I got a great deal on a Spintech muffler. It is 2.5”/dual in a s 3” single out. If you are familiar with the muffler, it has 8 “coils” in it. I love to Do a few full throttle on ramp runs a few times a week. I also will do some dorky hyper-mileage-ing on fwy trips. My mpg’s are lower than they have ever been. With just a Hypertech, I have got 17-19 on some fwy trips (factory 255 tires). Now it is low 14’s.

Initially I was flooring it a lil’ more often to near the noise, but not anymore.

I disconnected the battery for a reset too. Still the same.

Has this happened to others?
According to our resident exhaust expert Keith Carty, your system's 3 inch tailpipe is your issue. Call him at 1-760-877-4234 and tell him what you have. He'll explain what Toyota had in mind with the stock tailpipe size and give you some options.
 

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I did a muffler swap on a 2015 5.7L crewmax, all stock. I changed to a Magnaflow 12588 (which is a very quiet version) 2.5" dual in, 3" out because I wanted a little better exhaust note. I disconnected the battery to weld it all together. I went from 16.5-17 mpg to 14 immediately. After a while (6 months) the mileage is back up to about 15.5-16. I think the ECM may reset all of the variables and it takes a while of easy driving to get the mileage back up. Not sure this is true, but I have had to disconnect the battery a couple times and I get a big drop in mileage each time that seems to recover after a few months of driving.

Also, I had a terrible drone sound at 50-65 mph which I fixed by cutting off the tailpipe right after it goes over the axle and turns to the passenger side. Essentially, I cut off 21" of pipe. Drone fixed. There is a formula I posted somewhere in here to figure out how much to cut off, but if you get a drone like I did, 21" is the magic number for the Tundra.

Love the sound now and the mileage is back to normal so we are all good.
 

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Discussion Starter #8
I did a muffler swap on a 2015 5.7L crewmax, all stock. I changed to a Magnaflow 12588 (which is a very quiet version) 2.5" dual in, 3" out because I wanted a little better exhaust note. I disconnected the battery to weld it all together. I went from 16.5-17 mpg to 14 immediately. After a while (6 months) the mileage is back up to about 15.5-16. I think the ECM may reset all of the variables and it takes a while of easy driving to get the mileage back up. Not sure this is true, but I have had to disconnect the battery a couple times and I get a big drop in mileage each time that seems to recover after a few months of driving.

Also, I had a terrible drone sound at 50-65 mph which I fixed by cutting off the tailpipe right after it goes over the axle and turns to the passenger side. Essentially, I cut off 21" of pipe. Drone fixed. There is a formula I posted somewhere in here to figure out how much to cut off, but if you get a drone like I did, 21" is the magic number for the Tundra.

Love the sound now and the mileage is back to normal so we are all good.
Thanx Mikecol. Good info. Just what I did. Got a good deal on a muffler. Kept the stock tail pipe. It might be slowly improving since I initially posted.
Thanx!
 

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I am currently looking at the possibility of installing a custom cat-back muffler system using long Power Stick mufflers that are packed with sound deadening material and an H pipe. The inlet / outlet are 2.5" that can be expanded to fit factory 2.75" pipes, the tailpipes or turn downs will be no larger than 2.75" so I should retain decent back-pressure and not lose low end power. Here is a set of Power Sticks on a 5.7L Tundra.
 

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Update. After about 6 months, the mileage is back up to normal. I have probably put around 15-20K on the new muffler and am now averaging 17 mpg with 90% highway mileage driving at 70-75 mph. I do think the ECM resets itself when the battery is disconnected. Also, it is summer time so pat of the mileage increase may be summer gas vs winter gas, but my mileage dropped instantly to 14 mpg with the battery disconnect and welding work, and has slowly got better and better week after week until it is now back to normal.
 

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Discussion Starter #11
Hello. I put the stock back on (the Spin-tech I had installed on another factory cat-back in case I didn’t like it. Did an over 100 mile fwy trip Friday-Saturday, in LOTS of on/off crappy Los Angeles fwy traffic, mpg was 15.7.

I will revisit playing with my exhaust after I get some LT’s w/some Dirty Deeds planning.
 

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According to our resident exhaust expert Keith Carty, your system's 3 inch tailpipe is your issue. Call him at 1-760-877-4234 and tell him what you have. He'll explain what Toyota had in mind with the stock tailpipe size and give you some options.
This is the problem with most of the exhaust systems you find on Tundras... They aren't made by Keith at Dirty Deeds Industries.
I have the 8" Stainless with the resonator. It's fantastic.
 
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