I have all the mileage and gallons used on the Texas trip recorded (as with all trips), and have checked them over and over. I did say it was downhill dropping down from the Rockies into Texas, quite an elevation drop at very steady state driving. Also might have had a tail wind, lol, but the many hundreds of miles recorded don't lie. I think that air density at sea level does make a big difference in increasing gas consumption compared to being a mile high.
Just checked the numbers again, and for the major portion of the trip down I averaged 25+ mpg, and several stretches were 26-27, and one stretch of several hundred miles was 28. We averaged 24.5 for the entire trip (6857 miles, 280 gal), from Seattle to Galveston and back to Austin and then El Paso, including city driving in those cities plus Houston and San Antonio, before heading back up through the National Parks in Arizona and Utah and then back to Seattle.
Checking other numbers, for the trip down to see you in San Diego and back, we averaged up to 26.9 at about 75-80 mph. One stretch in northern Cal was 26.9 down off the mountains, 26 from there to Roseville, east of Sacramento. From Roseville to Bakersfield, the car also got 26. Back up along Highway 1 along the winding Big Sur road from Santa Barbara to SF, we averaged 24.7. In SF and north up the coast, the mileage figure decreased as we pooped along in tourist traffic along 101 in Northern Cal and Oregon. Not good there, was probably 22-23, thus bringing the mileage down for the entire trip (2875 miles, 118 gal) to 24.2.
One thing to remember, I have the back of the car just a little higher than that of the 91S, the front being the same, so that the car is slightly nose down in attitude. My aero engr studies indicated that this was slightly better for reducing vehicle drag at speed, but maybe only several percent.
Nowadays I get 21-24 mpg around the Puget Sound area, sometimes slightly lower, depending on traffic and where I drive, 22.2 for the last 1900 miles of city driving, plus a dash down to Portland and back ~300 miles).