Hmmm...

Couple of things come to mind:

1. The 8-speed automatic transmission is very responsive, which IMO is one of its best traits.
2. Your new tires are one inch wider. That alone is going to increase drag even if their overall diameter remained relatively unchanged. I’ve notice that fuel efficiency on these vehicles is very sensitive to tire width, which suggests they are very sensitive to changes in drag.

You might want to try manually shifting into 8th gear before it downshifts, to “train” the transmission. But I’d be very careful to avoid “lugging” the engine.

 

The OEM Bridgestone Dueler's on Tire Rack show 7.0" tread and 10.2" section width

The BFG KO2's on Tire Rack shows 9.1" tread width and 10.9" section width.

It looks like my BFG's are about 2.1" wider using the numbers above, so yeah that's a bunch of drag.

Not totally sure how to manually shift into a higher gear. Instead of Drive do all the shifting manually?

 

Additionally, your Bridgestones were 39# tires, now you're turning 57# tires. More unsprung weight and more rolling resistance with the new tread pattern.

 

Yeah true, but you gotta look cool rolling down the highway.

 

FWIW, when I swapped the OE 255/70R18 Bridgestone Duelers on my Sahara with a set of 275/70R18 KO2s, I saw a drop in fuel economy of about 1.0 - 1.5 MPG.

 

I tried the "manual" thing on my home from work yesterday. The dash says "M" and whatever gear you bump it to instead of "D" on the dash. So with the dash saying M8 at one point it's hard to tell if it's actually in 8th, or 7th and you're allowing it to go into 8th if it can. My point is I don't think you can force it into 8th from 7th gear.

I also noticed on way to work this morning on a flat surface, no wind in 8th gear, @ 57mph, tack @ 1,500rpm's, the current mpg is 25. But if/when transmission drops back down into 7th the current mpg drops to 16-18.

 

Part of it is tire roll resistance, which will increase on a wider tire. Plus the 3.45 gear ratio. The solution will probably to Re-gear. Holding it in 8th gear and lugging the engine, is not the answer..IMO

 

You have to be right on the cusp of the shift point. The 3.6 or 2.0 will stay in 8th with the larger 285/70/17's w/4.10s, even with the added weight on the Gladiator, factory Falkens or KO2's.. (as yours did the the Bridgestones w/3.45). Mileage in Gladiator, not so good, seems like I'm always hunting a gas station. Better in lighter 2dr.

 

My 8HP75 in my JLURD does the same. It will downshift at the hint of a hill while on CC, even after the Mopar lift and 37's. I was surprised that it would do that even with all the low end torque. Still does it and I added GDE tune. I just put it in M7 once I get to 55-60 and leave it.

 

This is a late observation to add but another factor I have noticed with my 2021 JLU 3.6 engine and 8spd ZF tranny is that wind has a serious and instantateous effect. The Wrangler in all its glory is about as aerodynamic as a dumpster. My tranny is silky smooth but if a mouse farts in my general direction, it drops into 7th or 6th. I have experimented with using Manual and selecting a desired gear and listening carefully for the sound of lugging and cross checking it with the instantaneous fuel consumption or fuel flow. In the end I decided to reduce the workload and just let the computer magic manage the transmission. It works beautifully.

 

I don't have an automatic transmission in my 2022 Willys Sport, but I am very surprised by those here reporting a serious loss of fuel economy in going from 8th gear down to 7th. I would think that the difference would be minimal.

In mine, with the manual transmission, I have monitored the instantaneously reported fuel economy on a flat highway while dropping from 6th (my highest gear) down to 5th, and it is barely if at all noticeable --- I'd guess that a very slight headwind, or the slightest uphill grade, would have much more impact than that gear change.