I changed the NGK plugs that came in the car with some Bosch BKR7E last year, and I seem to be down a mile or two a gallon. Do I put new NGKs back in or go for something like the Denso? I remember reading about better mileage and all a year ago, but cannot find the posts now. Are the Denso's really better? If so, I can get the K20TXR ND for about $3 a plug. The IK20s are about $10. From what I read, the only difference is the IK20 is one level cooler a plug. Just speaking of the Denso's is this worth it and why?