I recently put a Flowmaster 40 series SISO on my 05 F150. I expect gas mileage loss in the city beacause I like to listen to the sound. But I took it on a road trip and my mileage has gone down considerably. I am thinking maybe my o2 sensors are going out? Any suggestions?

 

its probably the 40 itself.. Those aren't the best for performance and gas mileage gains, they just sound good.

 

Same here. 40 series. Sounds great! Worse mileage all the way around. No biggy, just worse mileage. Didn't see any notable gains hp or torque.

03 f150 4.6L 150kmiles

 

I just installed a set of Ford Racing headers and that really big difference in torgue and gas mileage and sounds alot meaner.

 

Should have went with Magnaflow if you wanted MPG gains.

 

I wasnt necessarily looking for gains I just didnt expect loss. It sounds great though.

 

That would be the Slowmaster at work again!!!
Does it downshift more to maintain speed??

 

No, the only complaint I have is the mileage. I added a 77 series CAI today. It woke the whole truck up quite a bit. Especially the "slowmaster".