Hello, I've been getting gas mileage in the 20-22mpg range for a couple of years now. Car has about 65,000 miles and is all stock. Driven mostly to work, 24 mile round trip with cruising speeds or 40-60 mph. Some stop and go, but very little hard acceleration. Thinking of changing the plugs, which I'm almost certain are original...any thoughts or suggestions? Thank you!