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mine does the same thing but around 2000 RPM and my gas mileage is fine its right around 16 sometimes a little lower depending on how i feel like driving that tank or the winter gas and other factors
you should get atleast 12.... and the cat can be checked...hit it lightly with a dead blow and see if it sounds like a bunch of rocks in there if so replace it...
well seeing how i got a 5.9L, i get about 9 around town. (sucks but thats wat u get for havin a kicka$$ r/t)
lol.. thats not normal. 12city - 17highway is normal.. for a stock 5.9
i have a some stuff done to mine and i dont even get 9mpg. I get about 14 avg.
You should check your o2s also. having bad o2s can also f-up your cat. You should have ine right before the cat and one right after, very easy to take off anc check..
Also if you replace the cat, a highflow cat is good, but somtimes you may have to replace the sensors with a aftermarket brand, due to the factory sensors only working under factory cats flows.
or you can also get a o2 SIMM Fastman sells them, you buy one and it replaces your rear o2 sensor with a computer that tells your pcu you have a perfect flowing cat.
make sure it's not the exaust banging against the frame,
I had this problem when i got the truck, One of the front hangers broke and at low speed accel. it banged around and sounded like sh*t
yeh today i brought it into my auto class. i hit it and it sounded like loose debris inside. i also checked the temp on both ends. it was a lot warmer on th inlet side than th outlet side. so it seems as tho im gonna need to replace it.thanks