When I go there, it says the EPA window sticker mileage is indeed 21 US mpg with 17 city and 19 combined.
It does show the lower reading you are quoting as the 'new' epa mileage whatever that means.
It is amazing that someone with 33's carrying a load of camping gear gets better mileage.
My Jeep gets the same mileage empty or loaded, it isn't enough to measure if there is a difference and I compensate for the tires with the use of the gearbox to keep the rpm correct.
Don't forget one BIG one here, I have my engine tuned up 'manually' with no emissions controls robbing power. That gave me a nice 25% seat of the pants power boost AND a big mileage boost.
I also have to 'pass' tailpipe emissions so it 'is' burning clean! 15 ppm HC. .016% CO and 589 NOx.
Hmm, once again 'I' see where the EPA window sticker on your Jeep said 16 and 20 with 18 combined when it was new 'with' an emissions computer. 20 or real close to it is sure possible in a YJ that is 'manually' tuned up..
It got about that when I bought it with the donut sized tires on it. I figure that my 13-15 is about par for the course in a vehicle with oversize tires, a winch on the front, a tire carrier and full size spare on the rear, and 100lbs of crap inside that I regularly carry.
Again, I don't see much difference empty or full, it has the aerodynamics of a brick to overcome which seems to be the main effect in mileage.
My odometer and speedometer are GPS and 'measured mile' checked as accurate so my 'mileage' is 'real', not off for the tires. The 'perceived' mileage drops radically with oversized tires on.
Did you also change your speedo gear to make 'your' odometer accurate?
When I go over 65 mph, I get closer to your reported mileage for sure. If I run at 110-130 kph, (70-80 mph) I only get 18 max which is still OK.
If my wife only got 13 mpg in 'her' CJ7, there would be a riot happening with me fixing what was broken for sure. It happened on one trip, I was messing with the 'Nutter' directions one spring and had my carb at full rich like Nutter called for for a 'test' when I was a car passenger in a passenger side t-bone hit hard enough to give me a stroke among other things and I 'forgot' that I had the carb set rich. Wow, that cost us big time on one leg of a 3000 mile trip. I was out of gas at 250-265 miles per tank! We normally 'look' for gas at 350 miles... There 'was' a riot and in the ensuing tune up I found the carb stepper needles shoved all the way in.