The book says use premium but regular OK if premium unavailable so can I use regular all the time and sacrifice a second or two in my 0 - 6o mph. My mpg is better w/regular.
Your tag says that the vehicle is a Lexus 450h. That is a highly sophisticated piece of engineering that has been carefully calibrated for use with premium gasoline. Most truly modern and advanced high compression engines run best with premium. That’s why it’s a Lexus and not a Toyota. Of course, they also all have knock sensors that will adjust timing if inferior gasoline is put into it. So driving it with regular will not do any harm (unless your knock sensor goes kaput one day which is rare). For me the question is: Why spend $45K on such a refined engine and then feeding it junk gas to save $3 at the pump? But that is also none of my business 
Tom & Ray have by the way also weighed in on this issue: http://www.cartalk.com/content/features/premium/questions.html
Happy motoring!
If the manual says you can use regular, does it say only in an emergency, or for a short period of time? I have an earlier version of that engine in my ES300, and the manual says I can use regular, while premium is recommended. I’ve used regular for 10 years, no problem.
Back when “Regular” was 90 octane and “Premium” was 100 octane, a ten point spread, you can be SURE cars that required premium fuel would complain mightily (but seldom destructively) if they did not get it…
With today’s 87-92 five point spread, most engines that “require” premium fuel will tolerate regular if they have to without any destructive consequences…The 9.5 compression ratios just are not critical enough to bring on destructive detonation.
Yeah, because when you spend $45k+ on a car it would be folly to actually use the recommened fuel. Perhaps for my next vehicle I’ll buy one of those new Ferrari 458’s and then drive around with parking brake on at all times. [/sarcasm]
You likely aren’t getting better fuel mileage using regular either. Your engine’s ECU tune was written with 91 octane in mind. When you use fuels with lesser amounts of octane, then the ECU will retard the ignition timing to compensate, this will result in more fuel being used, less power, and possibly more emissions.
The compression ratio of the OP’s engine is 12.5:1
I dislike the labels of “regular” and “premium.” This suggests that premium is somehow better, which it isn’t (unless it’s what your car needs). But even if it’s what your car needs, it isn’t “better,” it’s “correct.”
In the end, you’ll do what you want, and probably be fine (unless you abuse the car). But, in the beginning, if you didn’t want to pay for high-grade fuel, you shouldn’t have spent money on a high-grade car.
Using regular will do two things only to your car. It will have slightly less power and it will get slightly less mileage.
[i] My mpg is better w/regular.[/i] Not likely. There must be something else going on. How are you measuring your mileage? It could be your lexus is having some sort of problem. Has any CEL's (Check Engine Light) shown up? It could be as simple as different temperatures with one tank than the other or the station may have switched over to winter blend fuels.sometimes using regular will also cause the coolant system to run hotter. using the higher octane fuel you will normally get better fuel economy and that would offset the higher price.
With 12.5 to 1 compression, Toyota is depending on direct injection and a four cam, four valve techno-marvel engine to avoid detonation, not the difference between 87 and 91 octane…
But this argument is academic…If you buy a top-end $46K techno-marvel vehicle, feed it what the book says…If fuel cost poses an economic hardship to you, you are living WAY beyond your means…
Maybe the OP feels the reduced response when using regular and subconsciously drives more gently.
I’m afraid I question your claim that your mileage is better with regular. The car is retarding the timing to protect the engine from damage when you use regular, which means worse mileage, not better.