I have an 04 Limited V8 4x4 103,xxx miles. It became my daily (40 miles/day all freeway @65mph) in August. I was getting 18-19 mpg on 87 octane. In October it started dropping. Now it's down to 13 mpg on 91 octane 12 mpg on 87. In May I changed: timing belt/pulleys, water pump, t-stat, spark...