..would'nt make that much of a difference due to weight of vechicle, the gas savings would'nt be enough to offset the fine if caught..not knowing particulars such as engine size, size vech, etc....would be hard to guesstimate mpg but thinking 1-2 max...newer vech with cats enginered better now..with puters on vech they rely on o2 sensors fore and aft of cat to adjust fuel/air mix with nothing to compare to you run risk of running engine to lean/rich and both bad . ....if you have state inspection and don't know the inspector you will have to reattach to pass...hate to say it; but my advise----grin and bear it..plus if you ever want to sell rv will probably need cat installed...sorry!! ..I would try to keep air filter, pcv, fuel filter,sp.plugs up to snuff...you can try K&N filter, pricy to buy but reusable, some applications help improve mpg..:D