I have a 99 4.0 auto 20 gallon tank on a 4 in lift and 32s, I do mostly freeway driving about 280 a week just freeway. I’m getting roughly 9-10 mpg. What can I do to get better fuel mileage. Sucks having to fill up once every 4-5 days with California gas prices

 

Check the O2 sensor . and is your transmission is going into 4th gear and the torque converter is going into lock up ?

 

Is the engine in good condition?
is the check engine light on?
how fast do you go?
what transmission do you have and what gear ratio axles?

 

The easiest answer is to take your foot out of it. What speed do you drive?

The most effective answer is to return the vehicle to stock. Remove things off the roof if you have anything there. Reinstall the air dam if you removed it.

The realistic answer is to ensure everything is maintained. Give it a tune-up. How is your air filter, spark plugs, plug wires, distributor, fuel system, vacuum system, emissions system, exhaust.

What is your tire pressure? Keep those tires aired up for better mileage.

 

I do have an exhaust leak so the second o2 sensor isn’t really working but yes the trans is getting into forth. I do about 70-75 down the freeway that’s roughly 2250-2300 rpm. Axles are d30 and 8.25 with 3.55 gears. Tire pressure is up to 45-50

 

Gearing will help as well. What gears are you running. 3.55's will get you better mileage if you are not running them now. A good friend of mine has 33" tires and is still running 3.55's and he is getting around 20mpg highway.

 

yeah i have the factory 3.55. Could it be the alignment?

 

45-50 psi? Wow!!

Does it ride like a pogo stick?

 

Stay at 65mph and I bet you'll get more miles out of a tank. The engine will work harder the faster it has to push a barn door down the highway.

Merge onto the highway gently. It is not a race.

Give the engine a full tuneup.

 

There's nothing wrong with your setup. 3.55's and 32's will reduce your mileage, but you should be closer to 15 than 10.

What's your driving style? Heavy accelerations are terrible for mpg's. Gentle on the pedals, gentle on the wallet.

I still think there is something to be gained from a basic tune-up.

 

How is gas mileage being calculated?
Is your speedo accurate? Ever checked with a phone app?
70 to 75 is gonna kill your mileage as others have stated.

 

For 33s you need either 4.10s or 4.56s for a good balance of power and efficiency. A 3.55 ratio on 33s will be a dog and the transmission will get fussy, especially on hills.

 

In theory this sounds good until you buy a lifted XJ with 33's & 3.55 gears and you get 20 MPG... This may be an anomaly but factual. If I remember correctly this member asked for advise on efficiency and the 3.55 are just fine for a daily commuter with 32" tires. I agree with checking the speedo gear and a tune.

 

A clogged catalyst can provide terrible fuel mileage.

there seems to be a big drop in efficiency at or near 60 mph speed and above. The effect of aerodynamics.

me, 33x12.5 mt tires, 4.11 gear, heavily loaded, I can get around 17 mpg at 60 to 65 mph, but at 70 mph the mileage drops to about 15 mpg. level highway, non stop. 2 out of 17 drop in mpg, near 8%.
Speed Kills.

 

No theory here. I've had both. I've had 33s on stock gears. I have 33s on aftermarket gears (4.56s) right now. Never hit 20 MPG in either scenario. I've also never seen a Jeep XJ with 33s hit 20 MPG so I'm skeptical still. Keep in mind people usually put bigger tires on their XJ and fail to change the speedometer gear and their MPG calculations as well as their speedometer are off. I have the correct speedometer gear for my 4.56 gear setup so I'm qualified to give MPG calculations accurately.

Regarding efficiency, 33s on 3.55s is NOT the most efficient setup. Not even close. The most efficient setup is what gets you the closest to stock drivability. This would actually be 4.27 in theory. In actuality, you either go with 4.10 or 4.56. Reference chart below.