I remember seeing a lot of posts about gas mileage not being that great in the jeeps , but I am quite happy with what I am getting. I just did a road trip of about 300 miles , doing an average of 75-80 miles an hour.Looking at my computer during the trip , my average was 21 MPG , and with the economy mode , it was hitting as high as 30 MPG.This is quite good for a full time 4x4 , although of course the in town mileage is nowhere as good.I still have to do some testing with in town driving to see what i get. I did do a few minor mods to the truck , giving myself an extra 5-7 HP or so , nothing too major , but am happy with my results so far..

I would like to up the power a little and the torque for towing , and was interested in the kenne bell or vortech superchargers , but of course neither company has them available anymore , so I might go with roller rockers , new intake , bigger TB , and headers , which should up the HP by maybe 20 or so.Of course this will affect the mileage , but I can live with that..

A lot of the posts I saw were saying the highway MPG was around 15-16, which is what I estimate I get for city driving.Anyone else have some numbers to compare the highway and see if I am above average or about average?

From your post, I'm not sure what year, and model your Jeep is, or what engine you have. I'd be interested to see what it is - never heard of a Jeep that can get 30 mpg in "economy mode." In my CJ, I have an AMC 360 V-8, which got about 14-15 mpg before I installed the Holley 4-bbl on it. Haven't checked it since then.

The mods you mentioned won't help your torque for towing. Even if you get an additional 20 hp, it will all be up at the top end of the power band, and that's not where you will be operating for towing. The single best thing you can do to help towing will be to install lower (numerically higher) gears in your differentials. Of course, that won't help your non-towing fuel economy, but it may improve your towing fuel economy, as well as increasing the torque applied to the rear wheels for towing. It willl, however, be somewhat pricy -- at today's prices, figure $450 to $500 per axle. If you do that, it would also be a good time to install and LSDs or lockers you might have been contemplating, as you will save considerably on labor costs.

Guess i forgot to put the other info in lol. Its a 1996 grand cherokee with the 5.2 (318).It is equipped with the tow package so is good up to 6000 pounds.The trailer I was looking at getting eventually is about 2900 pounds , it's a 20 foot ultra lite , so should have no problems towing it.

If it was an easy mod I would put the supercharger I have for my trooper on , but I am pretty sure it wouldn't fit without a lot of modding which I don't have the time to do anymore , that's why I am in the process of selling my trooper and have the jeep.The motor has some miles on it , but I know the engine as I have had a few 318 over the years and they are pretty much indestructible as long as you look after them.It does have good power , like I mentioned was doing pretty much 80 miles an hour and was cruising at 2200 RPM , still pedal travel left on it.

My next days off will be installing my remote start/alarm , and I am using one of the bus interface to avoid putting in a bunch of relays and diodes.after that I was thinking of a mild lift.

How did you do in math class? I just have to say I don't think there is enough power in the fuel we use to get that mileage here in Texas.
The damned computer mileage doesn't count when doing a mileage check. I'm all for good mileage but I highly doubt your post as being entirely accurate.
The only way to know for sure what the mileage is on a certain trip is to use a little math and do your own mileage analyses..
I can see one tank of fuel coming down out of the Rockies into the flats of New Mexico being better than most. BUT the next tank going back up into the Rockies will average the mileage correctly for you.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

well, I did drive across alberta and was averaging 900-1000 km on a full tank , a little tired right now so didn't convert to us lol. The highways there the average speed is what I was doing yesterday at 75-80 MPH , so around 120-130 KPH.With city driving I usually get around 600-650 to a tank , but that's a lot of short stop and go driving and of course the idling at stop lights.This trip I was curious to see what the computer was saying , and for the whole trip , other than the 2 stoplights on the freeway , was saying 20-21 MPG.For the whole trip I used 1/4 of a tank of gas , for almost 4 solid hours of driving.

I know with my work truck , I average around 16 MPG with a 6.4 twin turbo diesel in a F350.The tank on it is also a little smaller than my jeep.My trooper has a smaller tank , and because of the gearing and tires , I was getting around 14 MPG in there on highway , and if lucky maybe 10 in city.

As for gas I was using , it was from Husky(Mohawk) and their regular gas is same as premium at other stations being 91 octane.Hope that helps explain more , don't know what they use for additives other than 10% ethanol , but most people don't realize that there are only 3 refineries in western canada , so the gas is the same , just everyone has a little bit different additive to put in it.

And if your wondering what all I have done to the jeep so far , its minor stuff.I put in a cold air intake with a heat shield to keep the engine heat from affecting it too much , a 3/4" ring on top of the TB for a little more air , new plugs , cap , rotor , 80 thermostat , a different coil(hotter spark) and a larger exhaust.I think that's all I have done other than the LED tail lights and brighter headlights and new stereo , although the old memory isn't as sharp as you get older <img src="/forums/images/graemlins/scared.gif" alt="" />

Could tell you more about the trooper as it was a frame up build and most was total custom stuff on it.The jeep won't be altered too much from stock , too little time and space <img src="/forums/images/graemlins/smile.gif" alt="" />

Not doing the math, keeps you in the dark! A full tank = this or a full tank = that doesn't mean one damn thing.

You wrote: And if your wondering what all I have done to the jeep so far , its minor stuff.I put in a cold air intake with a heat shield to keep the engine heat from affecting it too much , a 3/4" ring on top of the TB for a little more air , new plugs , cap , rotor , 80 thermostat , a different coil(hotter spark) and a larger exhaust.I think that's all I have done other than the LED tail lights and brighter headlights and new stereo , although the old memory isn't as sharp as you get older.

The only gain I can find in that post are the headlights and the stereo! I have toyed with the tail light idea myself.. But since my Jeep is staying in the dirt I haven't spent the money yet. The rest of that stuff prolly cost you fuel mileage.

Your model Jeep came with a cold air intake.. Modify'n it WILL cost you mileage.
The TB spacer is known to do absolutely nothing. But a new stereo is good! Also I do like more light on the highway.

Just so everyone will read it again.. A hotter coil is only hotter at higher rpm than the stock coil will operate.. At normal rpm there is no gain.
Of course there is nothing short of a pressurized system that will ACTUALLY put more air into the chamber.. BUT think of it like this: If you did get more air in there the compression ratio would rise.. That would cause the valves to clatter.. which in turn would cause the knock sensor to shut the timing down to stop the clatter.. Now you have defeated the increased pressure you wanted by making the engine run worse than it otherwise would have with the expected compression ratio.

Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

I had a customer that would regularly get 19-21 MPG with his '93 5.2l ZJ on trips from Orange county to Mammoth Mountain. He had the Mopar Performance controller, the Mopar header kit with a larger single exhaust system, and the 1" spacer on top of the throttle body.

Big Jim, good info, but in the case of a ZJ, there is no knock sensor, and on the ZJ V8, a spacer on TOP of the the throttle body will help a bit as the stock plastic air horn is very restrictive due to the low hood height requirements.

Marty I guess we could argue all day about the "perceived" performance gains from spacers.
Spacers have served reliably on racing engines with carbs. The spacers of various sizes are used to contain standoff and also to gain or lose velocity by changing the length of the runners in the plenum.
HOWEVER placing a spacer anywhere in a TB system might make the owner THINK he had done something, but it is all pride of ownership.
You know very well that if there was any change in the volume taken into the chamber any of our modern engines would complain loudly! Knock sensor or not the effect of the volume change would be the same.
So no change in the volume...equals no change at all!
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

On a ZJ, the spacer helps to eliminate a large restiction ABOVE the throttle body, which would bring the wide open throttle air flow back in line with the same engine in a Dodge Van or Truck which does not have the restriction. A 5.2l in a ZJ was rated at 220 hp. In a Dodge, it had 230 hp. The difference is in the intake and exhaust restriction, the internals are the same.

Pumping losses are one area of fuel economy and power that can be lessened with a lower restriction intake and exhaust.
A spacer UNDER the throttle body won't have much effect, if any.

Ok Marty, in this discussion we should get the facts out there.
So MOPAR restricted the intake system on the ZJ. By doing this the ZJ is starving for fuel mixture? I wonder at what rpm the engine is starved out of that 10HP.
At all RPM is the engine running at less than optimum performance?
Does the lack of a "spacer" only allow the cylinders to completely fill at maximum or minimum RPM?
OR is that 10HP spread out from idle to max RPM?
I am completely befuddled why the highly paid engineers at MOPAR didn't catch this in all their extensive testing and computer programs.
Perhaps this obstruction is visible to the observer but isn't seen by the performance of the engine?
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />