I have a sound comming from the back every time i go over bumps and stuff like that.I went to buy shock mount kit,just wondering could it be something else

my car is running 17.2 mile per gallon on highway.what should i do to bring the miles per gallon a little higher

tune up the engine... plugs, wires, coil packs, 02 sensors, fuel filter, seafoam, run premium gas only, oil change...

That sound is most definatley the rear shock bushing, I believe you can just replace the bushings. like $20.