Here is a graph showing the actual mileage from my 2008 rh400x over the last 18 months. I keep receipts from each gas purchase and write down the reported mileage between fillups. I think this is as accurate as I can get in recording mileage. For demographic data, I live in central Massachusetts in an area with plenty of hills, I have a commute of about 5 miles each way to work which is what I primarily drive. I drive about 8.5k miles per year (pretty light, I know.) I have currently been using only 93 (or better) octane gas since Lexus calls for it. But I'm very tempted to switch to regular and see what actual impact that has.
I think you can see quite clearly when the changeover to winter gas happens.
I also have data on on (in)accurate the computer is at calculating gas mileage. It overall inflates actual mileage by and average of 5.4%, but ranges from 2-10% (but I'm not sure what influences that.)
At current gas prices ($2.65/g in my area) it will take me owning the car for 9.75 years before I save more in gas costs than I paid in a premium over the 2008 rx350. So, it wasn't worth it economically but I have to say the hybrid is more fun :).