Driving south on HWY 99 from Fresno to Bakersfield I decided to get some consistent numbers for a completely flat highway cruise. Speed limit is 70 MPH once you leave Fresno and until you get a few miles north of Bakersfield. Where the highway "dives" below overpasses I used manual throttle inputs to accelerate slightly on the descent and decelerate slightly back to 70 MPH by the top of the ascent, essentially keeping LOD (for those ScanGauge users out there) constant and minimize the effects of these slight terrain changes. Same technique was used for overpasses, which there aren't nearly as many of on this stretch of highway. Tire pressure was 40 PSI @ 90F "cold" since that's what my garage runs right now (roughly equivalent to 38 PSI at 70F). Outside temps were in the high 90's and the A/C was on the entire time.
I reset my trip gauge once I hit the freeway and at the start of each run after stabilizing at a test speed and over the course of 10 miles @ 65 MPH I saw 38.2 MPG. Once the speed limit stepped up to 70 I did too and reset the trip gauge and over 60 miles I saw 36.8 MPG. I then bumped it up to 75 MPH for a little over 10 miles and turned in 33.6 MPG. For S&G's I dropped it back to 65 for 10 miles once the highway had opened back up to 3 lanes and saw 42.5 MPG!?!? No hypermiling besides the constant LOD driving I mentioned earlier, no drafting, etc. I didn't see much in the way of wind (air quality was HORRIBLE last weekend in the Central Valley with wildfire smoke and smog, so winds were light). I'm not quite ready to believe 42.5 MPG on a flat at 65 MPH even though it is in family with the MPG improvement I saw between 70 and 75 MPH. The last 10 miles in to B-Field I bumped it back up to 70 and saw just under 37, same as earlier that drive.
I'm going to need to do some more sampling to validate what 65 MPH yields before I'm willing to stand by that 42.5 MPG number but I do have some ideas why it was higher than when I first got on the freeway.
-When I hit the freeway I'd only been driving the car for a few minutes so while the engine coolant was warmed up perhaps the engine oil wasn't quite there and I'm sure the transmission wouldn't have had time to get all of the way up to temp. The tires wouldn't be quite as hot yet either so their rolling resistance would be a little bit high as well.
-The interior of the car had been heat soaked prior to hitting the road so the A/C compressor would have been loaded heavier than it would be once everything is cooled 10-15 minutes down the road.
I continue to be amazed at how well the Si can do if you want to coax great mileage out of it.