i got 28 miles to the gallon when i went to galveston this summer with the bed of my truck full

 

14mpg combined 50/50

 

Does anyone know if a catback exhaust make gas mileage worse? When I bought my truck in 2003, I checked the gibson site and they were offering free catbacks for my truck since they wanted it for measurements. So I said what the heck. I don't know about power gains, but I notice I also get around 14mpg. It's a quad cab, but I have the 5 speed which I thought should get better gas mileage.

And the exhaust is a single side swept with free flowing muffler... The only other mod was a uni foam filter which goes into the oem filter housing.

 

It had no effect on my gas mileage, but I guess if you were on the throttle more to hear it then it could hurt the mpg.