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Bad gas mileage 10-11 MPG

Also do I need to unplug the battery and reset the brain or anything?
 
1/2 tank of gas later and same, 10 mpg. Blew a hole in my muffler this morning and replaced it with a Flowmaster. Could a crappy muffler affect mileage? Didn't' notice a power difference after the switch so I don't think it was choking it.
 
1/2 tank of gas later and same, 10 mpg. Blew a hole in my muffler this morning and replaced it with a Flowmaster. Could a crappy muffler affect mileage? Didn't' notice a power difference after the switch so I don't think it was choking it.

Not to likely but a clogged cat will.
 
torque converter clutch not engaging will do it for sure, since without the clutch you are relying on hydraulics alone. if you feel like the trans is shifting weird I would probably start there.

check for fuel leaks / damp fuel lines (no kidding)

use the key trick to pull codes and see if anything there that didn't trip the CEL

are you running a 180 or 165 thermostat? the computer tries to get the engine temp to 195 by running a rich mixture so that the cat converter works efficiently, if temp stays low the computer will forever be dumping unnecessary fuel. O2 sensor can do it too but not usually that aggressive

Pretty much ties into this, if its crapped out.
 
No, that exact one, on the thermostat housing.

Also do you know if your spark plug gap is correct?
Last year after buying the jeep it passed smog with flying colors because the previous owner spent a ton of money on it including the intake system so it could pass and she could sell it so I assumed all that would be in order. So I'll put that on the list with exhaust manifold and vacuum leaks. I sprayed with brake cleaner for vacuum leaks but think I may buy one of those $40 smoke kits on ebay.
 
I don't know why I fight the easy stuff first. I assumed the plugs were good because it passed smog a year ago, my name is Dumass. I'll report a half tank later.
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I hate to tell you this.
but with the 35's and the lift. 10 mpg sounds right.
my stroker gets 8-12 city almost 20 highway. Im running 32's
 
My last XJ had the same exact set up and was a Renix and I got better mileage so I have to disagree with you.
 
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Is your speedometer/odometer reading correctly? Mine reads 10 mph high with 35 in tires and 4:56 gears.
 
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