New flowmaster muffler and my gas milage is crap?

[Curb71]

I had the 50 series flowmaster muffler installed a couple weeks ago and noticed since then my truck is different. While on the freeway goin 70-75mph it used to run around 1500rpms and now its a little over 2k. This hill on the 118 freeway I go over constantly I would get around 12-13mpg goin up it. Now I get about 10mpg goin up it. After I removed the HC filter from my TRD CAI I was getting about 18mpg on freeway but now Im getting about 15mpg.

Did the muffler make my truck run different and kill my mpg or is there something wrong with my truck?

[Sweet Mercy]

Curb

I have installed two different exhausts on my 5.7. The first being a JBA which I personally did not like because of drone--again just my opinion on my particular truck and my set of ears. I then installed the flowmaster 50 series big-block which was better with the drone but I soon got tired of feeling like the low end torque was missing. Again my opinion on what I felt.

I liked that the truck, when stock, pulled soooo good in 6th gear and rarely downshifted to 5th on hills but when I changed the exhaust, the truck more quickly downshifted on the same hills, although it did seem to run better at higher rpm's. I'm not one for high rpm's a lot so I put my factory exhaust back on and couldn't be happier.

[Rifleman]

The only thing that is going to change your RPM at a given speed is if it's in a different gear (unless you changed tire size). In my opinion, the exhaust system on the Tundra is pretty darn good. Not very loud, but efficient. Sometimes, lowering the back pressure will hurt your mileage at constant lower RPM driving speeds. It will give some more power on the top end, but gives up some on the lower. Try disconnecting the negative terminal of the battery, leave it off for several minutes and then reconnect. This will reset the computer and let it relearn with the new muffler. It might help pick up your mileage some. I have the DC with towing option and my truck has never run at 1500 RPM at 70 to 75 mph. It's closer to 2000 to 2100 RPM. You might take a look at the welding job done also. If the muffler was put on at some weird angle that left a piece of the pipe wedged at an angle in the muffler, it could be restricting the flow.

John K

[millsp]

Also be sure your tow haul button is not on.

[svedspx]

Originally Posted by Rifleman

Try disconnecting the negative terminal of the battery, leave it off for several minutes and then reconnect. This will reset the computer and let it relearn with the new muffler. It might help pick up your mileage some.
John K

The battery disconnect gets my vote. My first exhaust had 3'' pipes. Changed to 2.5 and made a big difference w performance.

[Curb71]

The tow/haul button is not on. That was the first thing I checked. Im a truck driver and noticed thats kinda how its been running is like the tow/haul button is on.
Ill disconnect the negitive terminal for a while to reset the ECU. Ill check the welding job too.
thanks all for your help.

[Curb71]

I disconnected the battery for about 30 minutes and hooked it back up and took it for a drive and it seems to run like it used to! back to 1500rpms on the freeway and much better mileage on the instant readout thing. thanks !

[Jer]

Not the first time I've heard this. I would NEVER put anything with the name 'Flowmaster' on any vehicles I own even if it were free. That's just me though. I'm sure someone will flame me for posting but those that know DON'T use Flowmaster. Little late for you now.

[matrix77]

Originally Posted by Jer

Not the first time I've heard this. I would NEVER put anything with the name 'Flowmaster' on any vehicles I own even if it were free. That's just me though. I'm sure someone will flame me for posting but those that know DON'T use Flowmaster. Little late for you now.

Can you elaborate a little? I''m learning.....

[RudyDep]

Yeah, Id like to know whats wrong with the systems that Ive been using for 15 years !!!

[semcoll]

Originally Posted by Jer

Not the first time I've heard this. I would NEVER put anything with the name 'Flowmaster' on any vehicles I own even if it were free. That's just me though. I'm sure someone will flame me for posting but those that know DON'T use Flowmaster. Little late for you now.

Please elaborate. I'm in the collision business and don't profess to be educated in the world of exhaust flow, backpressure and the like. Did I waste my money by installing a 50 series Flowmaster on my truck?

[Jer]

Do some searching. Many many many years ago I did a custom 3" exhaust on my 1990 Eclipse GSX with some mods. The exhaust actually felt worse then my HKS cat-back. This just can't be possible I thought to myself. Did some searching and found some bad reviews on Flowmaster and how little they actually flowed. Took it to a different shop and had them swap out the Flowmaster for a Magnaflow and the car ran like a stripe-ed @$$ed monkey the way I expected. It's been my experience over the years that whenever someone starts a thread on ANY forum about a new exhaust making the vehicle seem slower more often than not it's a Flowmaster. If you search the world wide web you will see more examples of what I'm speaking of.

[semcoll]

Read this. It is not a Flowmaster issue, rather a rule of physics.

Flowmaster, Inc.