While I was waiting for my loaner Lexus to arrive, I sat down and picked up a pamphlet that had a small article suggesting that higher octane in their Lexus cars can give better millage because our engines are designed for it.

Is there any truth to this?

yes and no...

if your car needs a premium then putting a premium gas will give you the factory mileage and putting regular will rob some of that fuel efficiency. But if you drive a 1992 tercel that is ok with a regular, then putting a premium in it will not make it any better

Above is correct. If it's what the factory recommends, then you get the best mileage.

True. When you put a lower octane fuel in a car engineered to run on premium, the engine management software will retard the timing to prevent the engine from knocking. That will result in less power and poorer fuel economy.

BUT the loss in power is usually hard to notice.

i'm not so sure on how much it'll effect gas mileage tho

With a CR of 11.8, I'll continue to use recommended premium fuel. I want the full horsepower that I paid for.

all specs outlined were achieved using premium during testing. Use something else you may not get as advertised

^^ true that besides it's ONLY a $0.10 difference at most and at a 16 gallon fill up thats a whole extra $1.60. if you can't afford an extra $1.60 then go trade in you car for something more affordable. just my $0.02

While this is true in newer cars, I'm curious if the '92 tercel referenced reacts the same why. I would say that old cars without complicated engine management software are more likely to benefit more from high octane gas than newer cars would be. I can't back this up with any evidence, except anecdotal: my '88 cherokee ran much better on premium, and seemed to return 2-3 mpg more than the regular for which it was designed.

Older vehicles can have issues like thick carbon deposits that effectively raise compression or other issues that can cause knock/ping (or timing retard for those with knock sensors, which is most vehicles in the last decade or two anyway) with the "correct" octane fuel, and higher octane acts as a band-aid for that.... the proper fix would be to correct whatever is wrong with the engine.

A vehicle should always be run on the lowest possible octane that causes NO knocking or pinging (or timing retard with sensors that stop the pinging).

For the 2IS, that is 91 octane under normal conditions.

For most cars it's 87.

You'll see no benefit from anything higher if the car is running normally. And you will see some decrease in any or all of: performance, mileage, and/or engine life if you use anything lower.