I have an '09 Mazda 3S, 5-speed auto, 2.3L with 65,000 miles. All stock, driven mostly to work and back, a 24 mile round trip with cruising speeds between 40-60 mph. Some stop and go, but no hard acceleration (mostly). Gas mileage has been in the low 20's for the past couple years. Was thinking of changing the plugs, which I'm pretty sure are original (I've had the car since 2011). Any thoughts or suggestions would be appreciated, thank you!