[Taylor95]

I have seen a small improvement from doing a partial grill block. This has helped my engine reach operating temperature quicker in the cold weather and it runs slightly warmer.

I agree with a hotter thermostat being more efficient. Here is someone else's experience:

On an LJ I used to have, it gained 1 to 2 mpg (depending on temp/weather/season) from installing a Stant 205F thermostat (Stant part # 13352). That's a good example of a Stant 205F thermostat in a Jeep 4L engine improving gas mileage in a moderate to cold climate.

A 205F thermostat is helpful in moderate climates to cold climates because it allows engine to run at (or near) full 210F op temp year round (instead of under-heating at 185F to 195F). IME, this is a big help in moderate to cold climates and improves gas mileage, especially during Winter. Over the last 20 years, I've done this on 3 Jeeps and 2 cars in a climate that varies from moderate to cold. It also works fine in Summer where we usually have 75F to 85F days, and sometimes mid 90s. Occasionally it gets up to 105F here in Summer. I find a 205F thermostat helps a little on cooler Summer days, more help in Spring and Fall, and a great help in Winter. On our occasional hot Summer days it's no help, but doesn't hurt anything. I'm using a Stant 205F thermostat (part 13352), which is same as Napa 205F and Gates 205F (Stant makes those brands).

If you live in a frequently hot climate your engine is already at, near, or above full op temp (210F) with stock 195F thermostat. If you live in a hot climate, a 205F thermostat offers no benefit and would probably be a detriment. If you're in a hot climate, I recommend upgrading to Stant 195F Superstat (part 45359), which is same as Napa 195F Superstat (Stant makes Napa). It opens at stated 195F temp and maintains a more stable, consistent temp than most thermostats. A stable, consistent temp helps the engine's computer fine tune combustion for better gas mileage. Another great thermostat is Motorad 195F Highflow (part 2000-195).

If your thermostat is to hot for your climate, your fans will work harder, which increases drag on engine and reduces gas mileage. It takes more power to turn mechanical fan when fan clutch engaged, which increases drag on engine. It takes electricity to turn the electric fan, which makes alternator work harder, which increases drag on engine. Increased drag on engine reduces gas mileage and performance.

XJs in very hot climates will have their fans turning a lot no matter what thermostat they use. For that situation l recommend a Stant 195F Superstat or Motorad 195F Highflow thermostat (high performance thermostats) and passive cooling mods: such as a larger radiator, oil cooler, transmission cooler, and/or hoodvent(s). Passive cooling mods don't use energy and therefore don't decrease gas mileage. In fact, in a hot climate, passive cooling mods can increase gas mileage by reducing the amount of work the engine fans do.

Gas burns more efficiently at hotter temperatures, but you want engine cool enough so your fans aren't coming on. The ideal situation is engine running as hot as it can without engaging mechanical fan clutch or turning on electric fan. The fans engage at around 214F to 216F. Gas burns really efficiently at engine temp of 208F+, and especially 210F+ (IME). The ideal engine temp is anywhere between 208F and 213F (IME). That's hot enough for an efficient fuel burn, but cool enough so fans don't come on.

https://www.jeepforum.com/forum/f11/...5-ats-1551782/
I have also read experiences where people have resolved their bad gas mileage by replacing their 165 degree thermostat with an OE 195 thermostat.