I have a 98 ext cab F-150 4.6L.. I have had the truck couple years now with 75k miles on it currently.. I use to be able to get 14-16 mph in the city, now I'm lucky to hit 13, I change plugs every year, wires are 1 yr old & have a K&N filer I clean often.. Only thing I havn't changed is my O2 sensors, do these have a big impact on mileage & power?? How often do you change them??