Flowmaster increas mileage?
 

Does adding flowmasters to a 4x4 truck increase its gas mileage? If so about how much?

Possibly; if you get a new, higher-flowing exhaust, your backpressure is lowered. Backpressure helps your truck make low-end torque, but it also means that your engine has to work harder to expel the exhaust gases. So, putting flowmasters on might fractionally help your gas mileage, but not by much. A whole new exhaust system would further lower your backpressure and might also fractionally improve fuel efficency, but again, it will not be much.

the flowmaster should help over stock, but it won't be a dramatic gain, besides, there's always that testosterone factor, you hear the nice tone, and you tend to try to keep hearing it... but free flowing exhaust has been well proven to improve efficiency. That is what is behind the talk of 10-20% gain in power and economy by adding headers...

fellro is correct, technically the flowmaster should help gas mileage but usually you tend to rev the engine and use excessive throttle and mileage drops. At least for a week or two until you adjust to the new sound

The backpressure=low-end-torque theory is crap. You want NO backpressure. I have a flowmaster, but didnt notice that much of a gain. I get 12-13MPG no matter how I drive (80mph HWY or 40MPH in-town).