I took my sister to work today on this 30 mile straight stretch of highway that had a speed limit of 75 mph. I got up to cruising speed and set the cruise control. I then reset my average gas mileage to see how I was doing. I expected the gas mileage to be less than average at that speed. Imagine my surprise when I ended up averaging 24.5 mpg. I have seen it get as high as 25 in the past few weeks.<p>Ever since I started using Castrol Syntec 5W-30 full synthetic oil, my gas mileage has improved dramatically. Has anybody else seen similar gains?