If you can tell when ur at exactly 1/4 or 1/2 throttle your better than prolly everyone else on here. Most people will have an idea but a cheap vaccum gauge can also tell if there are any other problem and maybe its not your driving style.

 

I get 10-11 MPG with 33x12.50 Firestone Dest MT's, and that's a 60 pound heavy-*** tire. That's in city traffic, stop and go... with stock 3.55 gears.

A cracked exhaust manifold I imagine would affect something.. but I'm not sure about gas mileage.

I recommend investigating the fuel injectors and fuel filter. They're a highly neglected topic in the jeep community. I think everyone, for the most part, forgets about the existence of the fuel filter (especially since you have to get inside the gas tank to get at it). Anything that gets passed through it because it's attained tens of thousands of miles of processing gas (and depending on where you are will determine how clean it is... but no one can know unless they're monitoring it with an aftermarket setup) gets put into the injectors, seriously jew'ing up the spray pattern and how much fuel is actually being input into the air intake.

I actually just recommended this link to someone else. For the price of the mod I'm positive it will continue to pay itself off well into a very long run given it's inexpensive price to do.

http://www.ericsxj.com/flomax/flomax.htm

 

what parts would you clean

 

I replaced my coil/plugs/cap and wires about 10 thousand miles ago (maybe less than that). Already my cap looks like it could do with either being replaced or some sandingh off the old dieletric grease that dried up and oxidized from all this cold weather. Same goes for the plugs.
The coil connection looked fine however... and it looks like it's brass or copper. The aluminum cap and wires however didn't look so shiney. Apparently copper / brass will stay cleaner for longer periods.

Something else I recommend is a good cleaning of the intake manifold. I rebuilt my engine at 10K miles ago and oh my GOD was that thing nasty on the inside. It looked like there was thick dark oil-fungus growing in there. I scraped as much as I could get out with a knife (I used the flat side), but I still didn't get all of it.

I went and had the Sears fuel-system/IM manifold cleaner done for 80 plus taxes... man was that worth it. It cleared up ALL of that stuff. I have a smooth and dry IM now. That was well worth every penny. I imagine you could take yours off and spray it down with some sort of cleaner that would eat all that mess off.

What cleaner that would be... I don't know.

 

Seafoam and tooth brush

 

what is the difference between the tire height that would throw off the speedometer and how you are caculating the MPG. The 31's circumference difference from your stock rubber could be 10% or more

 

i do my mpg by how many miles i went devided by how many gallons of fuel

 

this is my 3rd XJ and somehow I thought the gas mileage was around 18mpg hwy...I think I'm getting like 10mpg right now

 

I was underneath the beast wiring in a wideband 02 sensor to use in tandem with my SAFC-2 when I found out my 02 sensor wasn't even plugged in. That would explain my constant 10MPG whether city or highway regardless. I also run a stroker with 24lb fuel injectors.

Weird thing is I never have had a CEL come on for it....