TLDR version: Long story short, the math is a lot more complex than "half the displacement should be double the fuel economy".
Long version: I think the important point here to make is that the OP's base concept that there is a directly linear relationship between engine displacement and fuel consumption is what is fundamentally at fault. Just because engine A is 4.0L and engine B is 2.0L does not mean that engine B will consume half as much fuel as engine A; there are just way too many other factors at play, not the least of which is that these are completely different cars with different purposes.
An 86 weighs roughly 2750 lbs (give or take), and a mid-90's Jeep YK weighs between 3100 to 3800 lbs. Some quick math here, but the Jeep weighs between 12-40% more than an 86. If we assume that lighter end of the Jeep weight scale is the I4 engine, you have two cars that are roughly within earshot of each other's weight (12%), and 20% apart in displacement, but the fuel economy difference is dramatically about 47% (17mpg for the 2.4L I4 Jeep to 25mpg for the 2.0L H4). If we go with the original premise on its own that there is a direct correlation between displacement and fuel economy and nothing else, then the YJ should get much better fuel economy then it does. The weights are similar, the engines are similar displacement, yet it is way off in fuel economy. Why? They're different cars with different purposes, so pretty much everything else is not equal. Gearing, how the torque and hp is designed to be delivered, aerodynamics, vehicle mass/weight, volumetric efficiency, you name it. So at this point I'm going to throw out the displacement theory because there is clearly more to fuel economy than engine size alone.
Looking more specifically at the 4.0L I6 the OP brings up, assuming the I6 is going to be the heavier end of the weight scale, we have a 40% heavier vehicle with 100% more displacement. Stated fuel economy differences are 15mpg for the 4.0L I6 vs 25mpg for the 2.0L H4, or 66%. Assuming you are using a 19 gallon fuel tank in the YJ, your estimated range is about 285 miles vs 330 of the 13.2 gallon 86 fuel tank; so your 86 should get you about 14% more range per tank. Now if we assume that the two engines used the same fuel (to eliminate the cost argument), the average US cost for a gallon of regular gas is $2.15, each car would cost you $40.85 and $28.38 respectively (44% more to fill up the Jeep). Now readjusting for reality, the 86's run on premium, so that tank actually costs $34.61 based on the current US average price on premium ($2.622). The I6 now only costs you 18% more for 14% less range.
Hypothetically if we drive both cars 1000 miles, the Jeep will use 66.66 gallons of fuel and cost $143.33. The 86 will use 40 gallons of fuel, and cost you $104.88. That's a savings of 37%.
I'm really not sure what the complaint is anymore. From all angles the FR-S/BRZ are doing better fuel wise than his Jeep, but being double the economy is a bit of a pipe dream if you are basing that solely on displacement. You are getting much better fuel economy, but you are also having to use more expensive gas, so while there is a lot of fuel efficiency gained, the increase in per gallon fuel costs negates the straight dollar savings if apples were really comparable to apples. Still, you are saving 37% of your money by driving the 86 over the same distance in your Jeep.
If that alone isn't worth it to you, feel free to send me that 37%.
