At 51,000 miles I replaced (for the first time) plugs and air filter on my 6 cylinder 4Runner. Before changes, I was averaging about 20.5 city/highway combined. After the new plugs & air filter, mpg when down to about 18 mpg. I replaced factory plugs with 6 Denso. It came from the factory with 3 Denso (passenger side) and 3 NGK plugs (driver side). The parts guy said the 6 Denso plugs would be the same as 3+3 as described above. Could this be the reason for decreased MPG? Any other ideas??? Thanks, Jim