wELL I'VE BEEN getting about 13 mpgs for awhile now. I have no cel's. My buddy hooked up his machine and 3 sensors came up on the screen that read healthy and 1 was flatlined wich was bank 1 sensor 2. the other 2 were fine. How many are there? i no theres 1 on each manifold but is there 1 before/after the third cat?

Four Sensors.

Two in front og the kitty cats (one in front of each cat).
Two behind the cats (one behind each cat)
Technically, the 2 after the cats are not O2 sensors in the full meaning of the term but are still referred to as such. Their job is to monitor catalyst performance and tell the pcm when the cat is bad or missing.

Just replace the two front ones... go to Max, he will hook you up!

thanks guys. my friend said they look healthy on his machine so i'll just let them be for now. What else would trigger my mpg to drop rapidly. I dont think it would be my wheels but i also wasnt paying attn after i installed them.