I drove 380 miles yesterday from the Seattle area to the Windy Ridge area (South side) of Mt. St. Helens and back. It was an absolutely beautiful day, starting out around 60 degrees and ending up around 80 degrees.
The drive down there was 60 MPH interstate freeway where the average speed was 65-67 MPH for about 45 miles, followed by 80 miles of backroads with speed limits generally in the 50-60 MPH range. However, there was a ton of road construction, meaning I sat and waited with the engine idling for probably a total of 15-20 minutes, followed by a lot of creeping along, and maybe 10 full throttle passes on two-lane freeways. I filled up right as I began, and filled up right at the turnoff to the national park road to Mt. St. Helens. 123 miles, 3.465 gallons, 35.56 MPG! I have never gotten more than 34 MPG, and that was with the OEM wheels/tires that were narrower, lighter, and a bit smaller in diameter (meaning the fuel economy calculation would have been more optimistic). That 34 MPG was also a similar drive, but in cool January conditions.
The second part was driving from the gas station up to the Windy Ridge viewpoint, twice, once for the fun of driving these incredibly winding roads quickly, and the second time to stop and enjoy the viewpoints. I filled back up at the same same gas station, at the same same pump even. 90.5 miles, 2.692 gallons, for 33.6 MPG. This tank was mostly 4th gear to the actual road that climbs to the 4100 foot viewpoint, followed by 3rd and 4th gear momentum driving in the 2500-5500 RPM range and high throttle openings on corner exits. On the way down, it was 4th and 5th gear, and lots of coasting.
The last section was all freeway home, a 40 mile section averaging 60 MPH with a few full-throttle passes, followed by a 60 mile section of freeway averaging 75 MPH, followed by a further 60 miles in freeway traffic averaging maybe 40 MPH all together. It was into a headwind, with the A/C running the entire time. 168.2 miles, 5.274 gallons, for 31.9 MPG.
Never had close to this kind of fuel economy since switching to the larger tires, which dropped the MPGs by 10% roughly. I did burn/lose half a quart of oil though. Several times, as I was slowing from rough 50 MPH to 30 MPH with third gear engaged to take a moderately high G corner (estimate 0.7G), I'd get on the gas at the apex and see a big puff of blue smoke. I could not recreate this but letting the engine idle for a few minutes, then blipping the throttle. I think this may be a PCV valve issue.
Otherwise, I'm truly amazed at the fuel economy. The engine is running even a bit more smoothly and quietly now after this trip. I've refilled the half-quart that I was down, will change the PCV valve, and then monitor oil usage again, because it didn't seem to burn any in the several hundred normally driven miles before this trip.