Need help Options

[Twilly]

Ok so i have a few questions about my car.

I have been tuning my car up to get better gas mileage and wondering if i need to do anything else. I replaced the plugs with NGK V-Power ones, wires, oxygen sensor, and fuel filter. I also changed the oil, oil filter, and air filter the other day. My question is if the o2 sensor before the cat is the only one i need to replace or should i replace the one after the cat as well?

Would resetting my computer also help with gas mileage? Also i have been putting injector cleaner in each of my full tanks of gas and I was just wondering which one is the best one to use?

Thanks!