2008 Implala seems to use a lot of gasoline - 2008 Chevrolet Impala

1. Change air filter
2. If you O2 sensors are bad or other emission sensor. You should be getting a check engine light if these are not working properly.
3. If you are sensing a loss of power, you could have a restriction in your exhaust system. I have seen catalytic converters plugged and mice plugging the exhaust. This causes the vehicle to lose a lot of power, if this is the case just have someone rev the engine while you feel the exhaust flow out of the tailpipe.