Hi to all,
As a new owner of an 08 Limited I am curious about the octane of fuel and what effect it could have on fuel mileage. While on a recent trip to Missiouri I ran two tanks of 85 octane regular fuel through the car and noticed that the mileage dropped from the hi 31s to mid to low 26s. After putting in a tank of 89 octane the mileage went right back up. Just wondering if this was a coincidence of was the enging trying to tell me something. Very happy with the new car so far. At 4000 miles no return trips to the dealership. Traded a 2005 Bucik LeSabre that had a terrible record, as far as reliability went. To be fair mostly centering around the Climate Control system early on but than the transmission started acting up. That ended the relatiionship.

Octane rating has nothing to do with gas milage. What that rating represents is how much the fuel resists detonation.
If you have an engine with somewhat high compression, you need a higher octane rating so that the fuel doesnt ignite before the piston reaches TDC. What will happen is that the compression forces will literally cause the air/fuel mix to ignite.
Now, its possible that your Avalon is intended to run on 93 octane. Its not all that uncommon for newer vehicles (especially ones with high-performance engines) to run on 93 octane for optimum performance.
You can run 87 octane fuel in these engines, but the computer will sense the lower octane fuel and will retard the ignition timing to compensate for this. The only real difference the driver will notice is a slight decrease in power and a slight drop in fuel economy. This could be whats happening in your instance.

As a former employee of an aftermarket additive manufacturer, I can tell you that higher Octane will indirectly boost your gas mileage by increasing your engines horsepower output. Octane boosting ingredients are key to improving fuel detonation efficiency which, in turn, increases engine horsepower output. As Toyota Fanboy explains, the more efficient the fuel detonation is, the better your engine will perform. So, your acceleration times to your desired speeds will be reduced causing you to use less fuel resulting in higher gas mileage. You can read one guy's experiment results using various Octane gasoline's in his car: http://www.weif.net/car/mileageoctane.php

Unless you engine has higher compression to where it can take advantage of a higher octane fuel, its a waste of money though...

Actually, it's all about the BTU value of the gasoline. Since the octane of gasoline is raised or lowered by varying the components that are mixed to make ASTM spec gasoline. ASTM specs do not specify what components are required, but rather what the "final animal" is supposed to do. Oddly enough ASTM does not have a spec for octane; however, they do have specs for just about every other thing.

Sorry for wandering; but, your gas mileage is a function of which components were used to make the blend that you are buying from your station. The total BTU's available is the aggragate blend of all the feedstocks used to make that gasoline. Odd as it may seem, you will actually get very slightly lower mileage by using a higher octane fuel. This is because octane boosters usually have lower BTU content per unit of volume. A good example is ethanol that is mandated today by the Feds for environmental reasons. Ethanol is an octane booster, so when the gasoline blend is put together that will later be blended with ethanol (they do this at the terminals and not at refineries), the actual blend without the ethanol is lower in octane than most engines require. But, when you add the ethanol, the blended mix (gasosline and ethanol) is blended to reach the pump listed octane. As you are probably aware, cars that burn ethanol fuel get 5 to 10% poorer mileage. This is becasue the lower BTU content ethanol dilutes the higher BTU content components of the gas.

So, to sum it all up, octane and mileage are usually inversely related.

Not to mention the fact that as expensive as gas is nowdays, youd be pretty foolish to run a higher octane rating than your engine needs.