We ran 87 in the 01 LC almost exclusively except some of the times we were very heavily loaded and in hilly terrain. Then we would run 89 or 91+. I noticed no change in mileage between tanks. Mileage consistently improved in higher altitudes (>4000 ft) regardless of the fuel grade.
When we got the 01 LX, the dealership told my wife it should use premium, even though it was no longer "required" in 01. So it got premium. I did some comparison with 87. Initially I could not tell any significant difference. But cruise control on rolling terrain was the first note of a difference. Driving the same hwy on 87 required more frequent downshifts by the cruise control on the incline portions; whereas 93 had notably fewer downshifts. This led to noticing other subtle differences in acceleration pedal responsiveness and shifting. Gas mileage difference was essentially negligible, with the LX getting slightly better than we did with the LC (LX= 14.3, LC = 14.0 overall).
Reading Toyota literature, all Toyota engines are made to adjust to the fuel in them and the engine controls will adapt. The adjustment is quick to a lower octane, but slow in recovering to tanks of higher octane fuel. Typically, it will take multiple fills of 93 to return to full efficiency. Comparing the same engine specs with Lexus showing 91 octane and Toyota showing 87 octane shows HP to be adversely impacted by <10 HP.
The cruise control comparison on the same stretches of hwy became a good test for other Toyota vehicles; and the frequency of downshifts on inclines was a factor, regardless of whether the vehicle was rated for regular or premium. Driveability and responsiveness in heavy traffic is also a notable distinguishing observation; again regardless of the fuel designation. Lexus is designed for smooth pedal response and minimal downshifts on the highways - for this 91 octane fills the bill. It will run on 87, but the pedal responsiveness will be a bit more jerky and downshifts on the highway more frequent. Neither octane resulted in engine knocking in any of our Toyota/Lexus vehicles; it is really about engine control and drive train timing. I usually would use 3 tanks of 93 before making the comparisons. So when heavily loaded, or in demanding situations, 91+ has a notable benefit. If you want a smoother, more responsive engine, then 91+ will have a benefit for essentially any Toyota engine in the past 20 years. If you want specifics, you can check the Toyota Training Course materials on the TIS site that cover engine controls. The takeaway for me was that Toyota designed their engines to run on a wide range of fuel octanes, so even those designed for 87 will respond with enhanced performance charateristics when using 93. So it is not as simple as Consumer Reports may lead one to think, the modern engine controls optimize performance of an engine based on the fuel it is using - thus affecting horsepower, torque, efficiency, and mileage.
Now that I am used to how our Lexus and Toyota vehicles drive with 93, the difference is noticeable when we use 87; even in vehicles designating 87. We only use "Top Tier" rated fuel.
