No it's not an old wives tale. They change the RVP each fall by using Butane which causes the fuel to have less energy. Along with a lot of other contributing factors such as letting the vehicle warm up, fluids being more stiff, snow causing friction, the dense air using more fuel, ect.

Heck, you are in PA. Says about the RVP gas right on the pump with a big old sticker.

And back to the topic, I have the Mufflex 24", no change, possibly slightly less MPGs.

 

I was getting 17 to 17.5 mpg in the summer. Now i have to baby it to get close to 14.5.

 

I increased my dyno numbers, but lowered my gas mileage. Could be the cold weather too. Will know more in the spring.

 

still have the mufflex for sale may be interested email me have a few questions todd@toddlyonsfishing.com

 

I haven't noticed any difference either way, which means that the MPG has probably slightly improved. Now that it's winter, my mileage should be less, but it's staying the same. If I had installed the muffler during the summer, I would know for sure. I guess I'll find out in a few months. I have the 14" on mine.

 

ok my question for you guys with the 14 inch mufflex system....do you here the exhaust at 75 mph with foot on floor in lowest possible gear....or do you here the engine???? my question is about the in the cab sound......

 

When I'm hard on it at 75 -80 MPH before it shifts into 3rd, it sounds wicked. At highway speeds, I can barely hear the exhaust inside the cab. When you give it a little gas, the MDS chug gets kinda loud and crappy sounding for a second until all 8 kick in, but nothing too bad. I don't have resonators on mine, and I have heard on here that removing them gets rid of a lot of the MDS chug. I took mine off the 4th day I had the truck before I got the mufflex, so I can't comment there. I was sort of disappointed at how quiet the 14" muffler was, I was expecting it to be a LOT louder. I'm glad I decided not to go with the 18 or 24.

 

I have the 18" and can say that once it breaks in it is exactly what i was looking for. When you are accelerating it gets plenty loud around the 1700-2200 rpm range. When cruising at 75 you can not even hear the muffler....it is extremely quiet. At 60 is where i notice it is a little louder but nothing bad, you can easily talk without raising your voice. Also if you are ever going to tow an enclosed trailer then I would stay away from the 14, the exhaust note bouncing off the front of the trailer is almost annoying in mine with the 18". Also I have the 3.55 rear end so at 75 mph my motor is running around 1900ish rpms and at 60 it runs 1450rpms.