I bought my truck new in 7/18. V6 TRD off road, automatic. After about 1500 miles my transmission settled in. Every 2 weeks I travel the same route to grocery shop and shop in general. Same stores, the same day of the week (most times anyway). Fill up at the same Conoco station.
The route coming home is a series of hills. After 5 months of traveling those hills in this truck while using cruise control, I know which hills rate 5th gear and which hills rate 4th gear to top, and about how long the truck takes to upshift.
So having some extra cash in my pocket, and my truck reading low fuel, I filled up with Conoco premium (93 octane, up to 10% corn pee content). The fill-up is in the very beginning of the shopping route. The truck now had 3800 miles or a bit more on the clock.
On the way home, I was surprised to find that the truck either downshifted less (meaning a downshift to 5th instead of 4th, or no downshift at all on lower hills) than when I used regular fuel from the same station. The truck also upshifted sooner after the downshifts. Since I get 3 weeks out of a tank of fuel (exciting life here), I got to repeat the test three more times, with the same result.
Same people in the truck. Same grocery load plus or minus a bag. Temp within 15 degrees, no snow or rain on the roads. Used cruise control all the time on the route home. The only difference was the fuel.
My train of thought, whether correct or not, is this. The engine has a high compression ratio (almost 12:1). It also has a knock sensor or sensors, which tell the computer to retard the timing in the event of spark detonation. Even with the direct injection giving a more thorough fuel-air mix in the combustion chambers, I feel that the computer is calling for a lesser degree of spark retard due to less sensing of detonation. This would possibly account for the decreased amount and duration of downshifting to top the hills.
When that tank ran low, I refilled with regular gas with the 10% corn pee content and the downshifts went back to the way they were before the test. Increased fuel mileage?? Possible. Any time the spark is retarded, mileage suffers. Worth the extra money? Probably not. Just passing an observation along.