Over the last 2K miles I've averaged 20.9 mpg with a couple of very recent 17-18 mpg tanks. During the same period (and distance) last year I averaged 21.6 mpg. Driving style and patterns should be essentially identical. Buying same brands of Top Tier gasolines (mostly Shell and Phillips 76) as I have for the 50K miles I've owned the car. So the injectors, valves, and upper cylinders should be clean. Air filter replaced (for second time) at 56K miles. So, with 60K miles on the car, I'm wondering if spark plug replacement is indicated even though the manual says 100K miles (but "more frequently" if "severe conditions" driving). My 700 lb trailer towing is <10% of total miles, I do very little mountain driving, but lots of driving at temps well over 90F. Any thoughts, comments, or experiences to share? Thanks!