Gas Mileage with a GFC Tanked

I'm having a very hard time coming to terms with my gas mileage... I have a 2020 Manual DCSB Off-Road with about 50K miles on it. I got it new with wrangler all-terrains on it and was getting about 22mpg tank average. I put a diamondback and ikamper over the bed which weight about 250 lbs and sat about level with the cab and I was still getting about 20-21 mpg tank average and even up to 22mpg tank average sometimes.

I sold that setup and picked up a GFC in October of 2022 and my gas mileage has tanked. I'm not getting about 16.5 mpg tank average. I know the GFC is a totally different form factor and adds a ton of drag and affects the aerodynamics of the truck but it's a similar weight to the previous setup. About 2 months ago I got new tires and put on a set of Open Country ATIIIs in 265/75r16. Slightly larger tire, slightly more aggressive tread, and about a pound heavier than the stock wrangler all-terrains, but I didn't notice any change in the gas mileage. I didn't recalibrate the dash but I compared the speedometer and odometer to a couple of different apps and the speed was pretty much dead on but the odometer was about 1.5 miles lower on the odometer than the app over about 30 miles. Didn't think this difference was enough to warner the $250 for the hypertech module and the actual size of the ATIIIs is about the same as the stated size of the stock wrangler ATs so I'm not sure if the tires are making any difference or the dash needs to be recalibrated but I've seen about the same tank averages with both sets of tires with the GFC on. The new ATIIIs are running at about 36-38 psi (rated for max psi of 44).

I also put a Sherpa Animas rack over the cab about a month ago hoping the wind fairing would help with wind noise and mpg, but I've honestly seen no improvement and the wind noise might even be a little worse.

I'm finding it really hard to believe my gas mileage is this bad and wondering if there's anything I can do to improve it. Apart from the tires, everything is stock. I've noticed some trips I can average about 20 mpg if I keep it around 55mph or below, but as soon as I get up to 60pmh or higher the gas mileage drops back down to about 16.5 mpg. I've seen others with the same truck and camper getting way better mpg in the 18-20 mpg range. I run 87 through it and I don't drive very aggressively so wondering what I can do to bump it up if at all?

Would running higher PSI help? I've had it as high as 41 psi and I didn't really notice any difference but it was only that high for a short period.

I'm also considering upgrading the suspension to a Bilstein 6112/5160 combo with an Icon leaf pack. Not really looking for much lift (1.5" max), mostly for stronger springs and better shocks. I know any lift is likely to drop mpg even more but is there any chance better suspension would improve the gas mileage?

 

You are doing everything wrong if you want decent mpg. The way to fix it is to take off all the weight, drag and downsize tires.
There is no magic bullet. If there were, automakers would already be using it.

 

Deep down I know this is the answer... just do not want to accept it and frustrated others are seeing better mpg

 

You added a brick to a brick.

And your previous MPG was exceptional based on threads around here.

 

It sucks but unfortunately the 3rd gen does not like carrying extra weight around. 16.5mpg is pretty decent considering what you're hauling. I average 15/17 highway/around town just by a lift, steel bumpers, and 285s. I'm also tuned. It's the fun tax as I like to call it.

Also changing suspension will not impact mpg unless you want to lower it. Might help then but then defeats the purpose of a truck. Lifting generally leads to even worse mpg.

 

16.5 isnt bad for what you are carrying. I hate to say it but a Ford F-150 would do much better on mileage.

 

Take what others claim their mpg to be with a grain of salt.

 

16.5 MPG is pretty much exactly what I'd expect with that setup. Toyo AT3s are really boggy.

 

Maybe think about switching the GFC for a camper shell, if you can live with that.

I have an ARE CX shell and I run E rated 265/76/16 Grabbers.

I average 20-22mpg on the highway. My truck is very light though, I have kept it armor-free because I do a lot of long distance rides. Like, thousands of miles.

Your GFC is obviously you’re biggest hurdle. Pic of my truck for reference on how it’s built:

 

Yup, driving around with a drogue chute on top of your truck and big tires pretty much does it in for mileage. RTT, upgraded tires, all steel underneath and on the sides put mine around 16mpg also.

 

Get gears and a tune.

 

My 5.7 Tundra did not like weight. 4K trailer was 10 mpg maybe.

 

Lol no.
Physics say energy required is energy consumed. Weight and drag require X amount of energy. No way to get around that simple concept.

 

You're right in regards to physics, but a poorly tuned engine that likes to downshift at the slightest molehill or crosswind that's now stressed under extra weight can absolutely benefit from a tune. Moving the shift points and holding gears longer can help recoup some mpg.
I'm speaking from experience btw. I went from 14 mpg average to 15mpg average with a tune. The gear hunting is terrible for gas mileage, which OPs trans is more inclined to do now that they just thrown hundreds of pounds of constant load over the bed.

 

I have a manual off road with 2”-2” lift
265/75 -16 ko2
Trd pro rim, the matte black ones, pretty lightweight
Up front is ome 886
Bilsteins 5100
Sdhq hidden winch
Warn zeon 10k
Dual agm batteries
Kdmax tune

I took off my sliders recently and saw about a 1 mpg gain

been getting about 17, according to the dash, per tank

 

I’m getting 17 in a stock truck. I think you’re doing just fine

 

If you're married to the GFC, I would reevaluate your wheel/tire combination. When I switched to the '20 Pro wheels (~2.5 lbs lighter than stock OR wheels) and Michelin Defender MS (~4 lbs lighter than stock Wranglers) I noticed a significant improvement in terms of lower rolling resistance and the ability to to hold 6th gear when when tackling a light hill or headwind on the highway. Those unnecessary downshifts punish your MPG. If you're not married to the GFC, think about a cab-height shell. Mine has had very little impact on my MPG.

 

When I swapped from a cab height bed rack/RTT combo to my GFC on my second gen I averaged 16-16.5mpg.

 

I get 16.5 mpg with my gfc and I’m a single cab 2.7 liter. I think your pre gfc numbers were a bit too optimistic. I lost a calculated 1.5 mpg with the gfc

 

Plus it’s winter and the air is more dense requiring more effort to push thru it. I see about 1-2 mpg decrease in the winter. If you were towing a 5100 lb trailer like mine you would see 9.5 to 11 mpg. Pulling a sail.