2004 WJ GC HO. Getting maybe 12mpg around town. Not smoking tires off the line . . . drive with an egg under my foot . . . but the coils are original with 172K on her. Should I expect improved gas mileage with new coils? And what else will new coils give me?

 

Trade 'er in for something with better gas mileage. You can put lipstick on a pig, but at the end of the day, it's still a pig.

 

I don't see what new coils would do but....
I remember when I sorted through most of the issues with my '00 XJ 4.0.
Then turned my attention to improving gas mileage.
Didn't expect much, and didn't see much, but figured I did what I could and it is what it is.
O2 sensors have a shelf life. Guys say about 150K.
So maybe try those.
Guys upgrade to 4 hole injectors and some report a slight improvement.
Somebody once said a long time ago, and I kinda agree with it, that it is not the 4 hole injectors that are causing any improvements you see.
Just the simple fact that you have fresh clean injectors in there that don't have 200K on them.
Good cleaning of the throttle body and IAC.
When is the last time it got new spark plugs?
I swapped over the diff fluid to synthetic.
Wanted fresh fluid in there anyway and figured what the hell.
Some say this might help.

 

I get the same with my Wrangler and RAM. Replace the coils only if they aren't doing their job, and you're getting misfires. You might be able to get better in small increments with what Ralph pointed out above, but if you bought the WJ for stellar fuel economy, you bought the wrong vehicle. It is what it is.

 

I've read that some folks have had good results from the Super Chips. Seems like all the money you could spend on "improving performance" could be better spent at the gas station

 

My stock 4.0 WJ on stock tires got around 16 mpg, what transfer case do you have? If it is the NP247 full time transfer case that is most likely your problem.

 

I'm not gonna chase my tail on this one. It is what it is and I love it so . . . "fill 'er up!"

 

With high mileage you have some blow-by there from compression loss. Try cleaning the throttle body. If the butterfly is held open by that ring of carbon, it will wreak havoc on how the TPS reads, causing all kinds of drive-ability and shift issues, ...since this is where your throttle plate spends most of it's time

 

That's the spirit! It's an old Jeep. It will never get good gas mileage. Ride on!