how can i increase the mileage on my 2008 h3 ?

https://www.fueleconomy.gov/

Check your spark plug have you change them, also the oxygen sensor and the Variable Valve Timing (VVT) Solenoid / Actuator those can have an effect on gas i do know when i changed my Variable Valve Timing (VVT) Solenoid / Actuator my hummer h3 had more horsepower. the one i got was the https://www.rockauto.com/en/moreinfo.php?pk=8353652&cc=1445526&jsn=411