Realistic MPG 4.0/AW4 33's with 4.56 gears

[Lordhelemt]

I'm trying to determine realistic MPG with a 4.0/AW4 running 33x12.5 and 4.56 gearing. This combo is actually in a CJ but since this came from an XJ I thought it would be best to check here. My son has been complaining about poor gas mileage (10-12) but I'm trying to see if anything can be done to get this to 15+mpg. Other than a basic tune up what else can be checked? I did just replace the O2 sensor (no codes but read about lazy sensors) so hopefully that will help.

Thanks in advance for any input.

 

[LK22]

I'm trying to determine realistic MPG with a 4.0/AW4 running 33x12.5 and 4.56 gearing. This combo is actually in a CJ but since this came from an XJ I thought it would be best to check here. My son has been complaining about poor gas mileage (10-12) but I'm trying to see if anything can be done to get this to 15+mpg. Other than a basic tune up what else can be checked? I did just replace the O2 sensor (no codes but read about lazy sensors) so hopefully that will help.

Thanks in advance for any input.

Thought I'd throw in what I could. I ran 33x11.5 with 5in of lift in an xj with stock everything else and 3.55 gears. Got ~18mpg highway and ~15mpg around town. Dropping down one size in tires and putting on a banks header/2.5in exhaust combo bumped my mpg by 2 on both ends. I know that's not directly applicable but it's data none the less. Furthermore, I've ran stock and modified xjs, none with your exact setup, but the absolute best you can do on a stock 4.0 is to allow it to inhale/exhale better, I.e. header, bigger exhaust, free flow muffler, and if u really want a CAI. Additionally, swapping out the outdated single pin injectors for 4pins (Cleggs are what I've used) and putting a wide band O2 sensor such as a NTK seem to help aswell. Also a dirty throttle body and surrounding sensors can hinder mpg. Goodluck!

 

[Numbers]

For my setup (35s and 4.88s), speed is big. With the gearing, the engine really starts to sing above 65mph and the gas mileage drops off a cliff. Not to mention the brick like shape.

I don't have any numbers, but 5 mph off of the freeway cruising speed could be a big impact. Just general driving habits can be a few mpg.

 

[Lordhelemt]

Thanks all. Will do some highway runs at a lower speed and see if I can find the sweet spot

 

[XJlimitedx99]

My son has been complaining about poor gas mileage (10-12)

What're his driving habits like? 10-12 sounds reasonable for moderate/heavy accelerations and running 75+ mph on the highway. If he's driving with a real light foot (slow accelerations) and sticking to ~65 mph on the highway and still getting 10-12, I would think there is something wrong with the rig causing the poor efficiency.

How are you calculating mileage? You'll have to compensate for the tire size and gearing change if you're hand calculating the distance per tank using the odometer.

 

[Lordhelemt]

Just filled up a will run another tank (just me driving) and see what mpg I get.

Yes the speedo is pretty accurate vs gps.

What rpms are you running on the highway? I’m around 2520 at 68 mph

 

[RCP Phx]

Just filled up a will run another tank (just me driving) and see what mpg I get. Yes the speedo is pretty accurate vs gps. What rpms are you running on the highway? I’m around 2520 at 68 mphthat sounds like you haven't

That sounds like you haven't done a speedo correction!!!

 

[Lordhelemt]

That sounds like you haven't done a speedo correction!!!

Speedo is showing just under 70 and Waze is showing 68. Keep in mind this is a CJ so there really isn't a tick for 68 mph. Do you think my RPMs are too high or too low? I suspect they are a little too high given my 4.56 gearing. I'm trying to see where the sweet spot is for MPG and also trying to determine if the AW4 is shifting properly into o/d.

 

[Root Moose]

In a CJ at ~70 mi/h I'd take 10-12 miles per short gallon as a gift.

CJs do not have aerodynamics - they are a blunt object torturing the air at those speeds.

Run a tank at 55 mi/h to see what you get. I'd think ~15 mi/h or even better could be possible if your axle ratio is not too low for the drivetrain combo to run at/close to BSFC.

 

[XJlimitedx99]

check out this calculator to play with gear ratios, RPM, and speed:

Gear Ratio Calculator

Looks like you're not too far off based on what you're saying. At 68 mph, you should be running 2521 rpm in OD with a tire rolling radius of 31". I know you said 33's, but this calculation uses the true rolling radius height value of the tire with the weight of the vehicle on it. 31" made the numbers look right.

 

[DaJudge]

I am currently getting 11-12 in town and 13-14 on the highway.
When I had the KO2s I was getting 13 in town and 15+ on the highway.
The STTs are a lot heavier and hurt my mileage more than I thought they would.

 

[Bent]

Also, wheel material and diameter will have a varying affect on rotational mass; steel, cast or forged aluminum. Forged being the lightest. I benefit from forged rims, but really suffer from the Toyo 35/13.5 MTs. (4.88 gears) I know weight and rolling resistance are killing me.

I have NO desire to figure my gas mileage.

 

[Black_Black]

Just an idea of the top end for these things - my Renix (4" lift, 33x12.5 ko2's, 3.55) gets 16-17 around town and 22-23 highway. My trip record is 25