I've only put a little over 1,000 miles on my 400h, but so far averaging about 26.5 mpg in temps ranging from teens (mornings) to mid 30s (afternoons). I have used premium, mid-grad and regular and I have noticed no difference in economy between the three.

I do suspect that winter temps and winter gasoline formulations will tend to reduce economy overall, so I'm hoping for better MPG in the spring.

 

We are getting about 3 mpg better with mid-grade versus the tank of regular the dealer put in it. (Now getting 26)

 

to the OP sounds about right to me. i woudl inflate to 35psi during summer and 37-38 during winter as well to help out a little re gas milage.

 

I've been using regular 87 octane this fall/winter and noticed no decrease in mpg compared to premium 92 octane in past winters.

 

The heating value (Btu/lb) of 92 and 87 Octane gasolines are basically identical, so there is no reason they should result in different mpg. Many tests have verified this. Ethanol has 2/3 of the HV of gasoline, so obviously it will lower mileage, depending on the percentage in the gasoline.

 

hmm, i am tempted to try 87 again.

 

There is at least one reason why mpg will suffer with lower octane. Lexus recommends 91, and if lower octane is used the ignition timing is retarded under load to reduce pre-ignition knock.

This is why power is also reduced with lower octane gas. As per the Lexus hybrid interactive guide:

3. Ratings achieved using the required premium unleaded gasoline with an octane rating of 91 or higher. If premium fuel is not used, performance will decrease.