I recently took a trip to Colorado for vacation and some 4-wheeling. On the way I was only getting 16-18 MPG @ 65-70MPH. Once I arrived in Colorado I filled up with 85 octane gas. For the next two tanks I got over 20 MPG (21.5 on one tank and 20.3 on the other). I did a couple of 4wd trails, (on both tanks) that were a few hours long and all the roads were pretty steep so a lot of low gears were used. I really expected to get worse MPG, but it was the opposite. On the way home I filled up and did one more 4WD trail. The trail was 2.5 hours long and the round trip for that was 70 miles. The next day I headed back home. I made it 385 miles and put 18.7 gallons in the tank. I filled up in Colby, KS. This is where it gets weird. I was lucky to get 300 miles on the next tank, and the same after that. Any thoughts as to why the better MPG at a higher altitude with 85 octane?  This sucks because I was getting 100 extra miles per tank.  

Oh, by the way I have a 1994 YJ 4.0L 5-speed 3.73 diff 32" tires.  New O2 sensor, comlete tune up less than 1 month ago.  

That one beats me, I can only dream about that kind of mileage, I'm lucky to make it 150 miles on a full tank in the 03 Rubicon. my final gear ratio is about the same as yours. even when it was new in the stock form it only would make it about 225 miles on a full tank.
  My diesel suburban gets better mileage than the jeep while pulling the 30' desert fox toy hauler, and that's with the wife following in the jeep. Hmm.
 Maybe that bring us back to the experiment that one of the guys was doing, by restricting the air box air horn more than the stock horn already does. goes with the thin air from the high altitude.  

check out this post.....
http://forums.jpmagazine.com/70/6352462/jeep-engine/05-6cyl-air-horn-size-needed/index.html  

Your computer should be making the mix leaner to compensate for the decreased oxygen at altitude. So although there's less efficiency at altitude due to lower O2, your computer is taking care of much of that; and you were using gears rather than power. The air resistance at altitude is less, but you were going slow so that doesn't count. But it does count running at 65 to 70mpg in KS. It was probably also hotter, and I don't know about anyone else out there, but I get better mpg when it's cool - denser air I guess. So maybe what you did in Colorado was get rid of air resistance - a killer for jeeps at anything over 50mph or so (ever notice how every standard car has a 5 to 10 mpg difference between highway and city, and jeeps have maybe 1 mpg?), add relatively cool denser air for the adjusted mix, lean down the mix, and be efficient - use gears instead of power. Maybe the difference between going and coming was uphill, overall, vs downhill overall. I feel that just going from BHM, AL to the gulf vs coming back, and there's only 800ft difference in altitude, but it's downhill in one direction and uphill in the other. You were basically climbing all the way there, and running downhill 2000 ft back.

My 2c and probably worth every penny.  

Pemburung; almost forgot about the spark plug gap therory.
maybe we should try gapping the plugs wider and get rid of the air horn for improved power and mileage.
It's scientific, you know.
you're right, the computer compensates for thin air and restriction.  

jerk  

I don't get those of you who complain about poor mileage with your rubicons!  fuel mileage in a jeep is all about pushing a brick down the road.  I drive an 18 wheeler for a living, and I drive my rubi in the very same manner. Loaded with about 400lbs of camping gear up top, and 15 gallons of liquids in jerry cans, etc etc, out back, with 265]75's,  I can pull 19mph on the highway. Back and forth to work, 22 miles each way, once a week I average 18 mph.  All I do is keep it under 68mph, and keep my RPM's in the lower end of the power band at all times.   Of course, back and forth to work is country roads, with only 3 stop signs.  As far as the original dillemma, I kinda like the up-hill, down hill theory.  In the 18 whlr there is as much as a 20%  differrence in mpg going generally up and then back down on a long trip. later-on  

I would believe those MPG numbers if you were talking about the diesel Liberty.
for me the poor mileage isn't a complaint, I didn't buy it for a comuter, It's a play toy. Very much like a gocart after parking the "parkinglot" in the drive way. It has a 550 cat, the computer says it gets 5.70 mpg average, between fuel stops. it is actually 4.35 average, going downhill it sometimes gets up to 100 mpg.
if your spedo hasn't been recalibrated, the rubi gets better than that because the spedo is slow from the larger tire size.
the MPH thing sounds about right though I can pull a hill about the same speed as the jeep in the truck. It weighs 46,000 empty.  

Yes,  I was surprised as well with my MPG for the rubicon,  but yes, I  do get those numbers,  I neglected to mention that I installed a shut off  toggle for my A/C.  My numbers are  with the A/C completely shut down.  With the factory shut off switch I used to only get 15 to 16 MPG, as I discovered the factory switch never really turned off the A/C compressor.  I also run a cold air intake, K & N.   As far as our liberty CRD,  my  god,,  my wife pulls 29 to 30 MPG on road trips, and 25 to 26 MPG  around our village.  I am very impressed with the CRD, lots of torque, and should last for several hundred thousand miles.  We just keep our foot out of it.  Dude yur mileage with that 550 kity really sucks, but then I run  the Northeast, our hills are just that, hills.  Even the adirondacks are mild grades compared to what you do.  I hall paint, 78,000 plus loaded 90% of time, (we have good backhauls from all points)  and I average 5.9 in summer 5.4 in winter.   Detroit, turned back to 475.   keep on trucking man, have a safe trip.   lateron;;  

What kind of paint do you hall that weighs so much?  

Yep, I'm in the northwest I-5 corridor, all hills.
I never thought about putting a kill switch on the ac. I hauled a bunch of libertys and was impressed with the tourque also and have never even driven one on the street, just across the lot and on the truck..
as for the paint question: 104" wide and 53' long semi trailer hauls a lot of gallons of paint at around 8-10 pounds a gallon. the weight is just a guess based on the weight of a gallon of diesel.  

ah,     paint weighs in @ no less than 11lbsper gallon,   some of the heavy duty industrial stuff , traffic paint, epoxies, etc etc cam weigh as much as 14 lbs per ga.   and we run no llarger than 48/102s.   If you are up and down I-5 corrider, you have seen our trucks,  in fact I got a nice manufactures discount on the stripped rubi, out the door for 21500, cause it is covered with sherwin wiliams paint.   The only thing I do not like about it is my having to put a kill swtch on the a/c.  Chrysler switches are poor at best, and I can see that way down the trail, they all will fail. but hey,,, its a jeep.  lateron  

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I'm new to this forum site but I cna't help but to ask if any of you guys use feul additives in your Jeeps, or other vehicles? A buddy hooked me up with some Biofuel stuff and I've run 4-5 tanks in my 1997 Honda Civic with 170,00 miles, and it's gone up from abour 34MPG to 38MPG. Check it out. 4X4mpg.mybpi.com I know it's some "Join me now" thing, but I read past that and checked the stuff out. I bought the stuff from a buddy and it works.