Bamafan1, you bring up a universal issue with our vehicles. If the 33" diameter tires went on your Jeep without a change in axle ratio gearing, the effect could be damaging to fuel efficiency. First, it's important to note whether your speedometer is accurate with the oversized tires, which create an additional "overdriving" effect. I did an article on the gearing subject for those interested:
To begin, is the speedometer accurate? Also, in town driving is a very difficult way to assess fuel efficiency. To determine "normal" mileage for your Jeep TJ Wrangler first requires a baseline. This would be a steady run on the highway, optimally flat and without grades, for optimal fuel efficiency.
The change to oversized tires often drops fuel efficiency. Your Jeep came equipped with 30" or so diameter tires (31" for the TJ Rubicon). Most manual transmission models have either 3.55:1 or 3.73:1 gearing, which you will find noted on the window sticker or at the axle tags. Either of these ratios with a manual/overdrive transmission is considered "tall gearing". This is low emissions oriented gearing and supposedly helpful to fuel efficiency. However, the low engine rpm on the highway at cruise speed in overdrive is anything but efficient.
As a rule, engine efficiency is best at or near the torque peak of the engine. On the 4.0L inline six in your Jeep TJ Wrangler, peak torque is around 3000 rpm. If you notice while driving, your engine rpm at a 60 mph cruise in overdrive is more like 2000 or less rpm.
Fuel efficiency might drop even further with oversized tires. Oversized tires, instead of adding a useful additional overdrive effect, actually drops fuel efficiency. Why? Because the engine is even further off it's peak torque rpm and under an increased load due to the oversized tires! To restore the engine rpm on our XJ Cherokee to stock (similar to stock tires with the OE 3.55:1 gearing), I had to install 4.10:1 gears. This gearing corrected speedometer calibration, too.
So, to restore fuel efficiency with 33" tires, even to the stock level, you need at least 4.10 gears. Frankly, in my experience, with all the added weight of accessories and add-ons, I should have gone a step further and switched to 4.56:1 gears on the XJ Cherokee. Many go to 4.56:1 on the Jeep YJ or TJ Wrangler with 33" tires and a manual/overdrive transmission. We take a different approach with the 3-speed automatic transmissions to 2002.
So, confirm your axle gearing and speedometer accuracy. If the speedometer is correct, let's look at the engine rpm at 60 mph in 5th gear/overdrive. (I keep assuming you have a manual transmission, is that correct?) Once you note the engine rpm at 60 mph, we can talk about ways to improve performance. As for your concern for an engine misfire, one issue for misfire can be a lugging engine—too low an engine speed under load and not efficient enough to burn fuel properly.
I did take a recent initiative with the XJ Cherokee and was very happy with the results. Though historically shy of engine programmers and PCM tuners, I chose to test a Hypertech performance software upgrade. Set for unleaded regular fuel, the single most impressive gain was that my 4.0L six, similar to yours, now delivers peak torque at 2000 rpm, a full 1000 rpm less speed than stock! This means that I can cruise in overdrive at 60-70 mph without lugging the engine. In fact, with only exhaust system improvements that were already in place, both the engine's performance and fuel efficiency jumped up. See my article and details at:
This actually was a surprise and real compliment to Hypertech's engineering. There is no "ping" with unleaded regular gas under load, and the XJ's curb weight is hefty! I would not recommend a software tuning remedy if an engine is worn badly or out of basic tune. At the current 145K miles on the engine, which is in good condition still, the gains were substantial, including fuel efficiency and much improved acceleration across the power band.