As the fairly new owner of an ’04 UZJ100, I did some searches of this forum for collective wisdom on improving gas mileage in the 100 series. I also searched independent (non-manufacturer) web sites for detailed information. Disagreements about what helps and what doesn’t help are common, but there are some things that seem to make a difference.
The U.S. Dept. of Energy, U.S. Environmental Protection Agency, and Consumers Reports have 14 actions and the respective mpg improvements (or not) on their web sites:
1. Careful driving instead of aggressive driving: up to 33% improvement
2. Driving the speed limit instead of faster: up to 23% improvement
3. Turning engine off if idle time will be more than 2 minutes: up to 19% improvement
4. Maintaining the engine and vehicle: up to 4% improvement
5. Removing excess stuff carried: 1%-2% improvement for each 100 lbs removed.
6. Using the correct weight lubricants: up to 2% improvement
7. Using low rolling resistance tires: up to 2% improvement
8. Maintaining the correct tire pressure: up to 0.3% improvement for each 1 psi
9. Using cruise control: variable improvements
10. Minimize driving with a cold engine/vehicle: variable improvements
11. Removing unused roof racks and other things carried outside vehicle: variable improvements
12. High-flow air filters: no improvement in mpg but potentially better performance
13. Rolling down windows instead of using A/C: no improvement on the highway, variable improvements at lower speeds
14. Gas mileage improvement gadgets: no improvements and potential fuel system or engine damage (from link to Federal Trade Commission web site)
In addition to the list above, these are specific things that may help gas mileage in the UZJ100 (from forum searches related to gas mileage and maintenance items).
Likely to help:
Oxygen sensor replacement
Catalytic converter replacement
Correct wheel bearing preload
Throttle body cleaning
Mass air flow sensor cleaning
Engine control module reset
Vacuum leak repair
Spark plug replacement
May or may not help:
Spark plug brand/type
Gas brand
Gas formulation (except for high ethanol content gas, which lowers gas mileage)
Gas octane
Installing a tuning chip
Time of day gas fill ups
Differential oil brand
Engine oil brand
The U.S. Dept. of Energy, U.S. Environmental Protection Agency, and Consumers Reports have 14 actions and the respective mpg improvements (or not) on their web sites:
1. Careful driving instead of aggressive driving: up to 33% improvement
2. Driving the speed limit instead of faster: up to 23% improvement
3. Turning engine off if idle time will be more than 2 minutes: up to 19% improvement
4. Maintaining the engine and vehicle: up to 4% improvement
5. Removing excess stuff carried: 1%-2% improvement for each 100 lbs removed.
6. Using the correct weight lubricants: up to 2% improvement
7. Using low rolling resistance tires: up to 2% improvement
8. Maintaining the correct tire pressure: up to 0.3% improvement for each 1 psi
9. Using cruise control: variable improvements
10. Minimize driving with a cold engine/vehicle: variable improvements
11. Removing unused roof racks and other things carried outside vehicle: variable improvements
12. High-flow air filters: no improvement in mpg but potentially better performance
13. Rolling down windows instead of using A/C: no improvement on the highway, variable improvements at lower speeds
14. Gas mileage improvement gadgets: no improvements and potential fuel system or engine damage (from link to Federal Trade Commission web site)
In addition to the list above, these are specific things that may help gas mileage in the UZJ100 (from forum searches related to gas mileage and maintenance items).
Likely to help:
Oxygen sensor replacement
Catalytic converter replacement
Correct wheel bearing preload
Throttle body cleaning
Mass air flow sensor cleaning
Engine control module reset
Vacuum leak repair
Spark plug replacement
May or may not help:
Spark plug brand/type
Gas brand
Gas formulation (except for high ethanol content gas, which lowers gas mileage)
Gas octane
Installing a tuning chip
Time of day gas fill ups
Differential oil brand
Engine oil brand
. 
