There are a lot of valid points here, and here is something else to think about. I don't know how true it is, but a service mgr I talked with told me, so here goes...
Yes, you have knock sensors that change your timing to help control the possible pinging when using a lower octane. Sounds great! Only problem is, that is very hard on the sensors and I hear they're very pricey to replace. Another customer was there that has had to replace them, and wouldn't tell me how much, just that I didn't want to have to replace a sensor
I believe you can use a lower octane, but it has it's consequences...
1. Less than optimal performance and fuel economy, which will have you filling up more often anyway, possibly costing the same as using premium.
2. You might have to replace $$$$ sensors. Will Toyota know, and not cover them? Expensive chance to take.
Make 1 less trip to McD's or Starbuck's once a week, that should about cover the difference in gas. People will pay $5 for a 16oz. cup o joe, but are freakin over 10-20 cents more for a GALLON of gas that'll get you a helluva lot farther than that Big Mac, or coffee.
Let's make it simple. Take you avg # of miles per year. 13200 for me. Now divide that by mpg, lets say 17, (I've been getting 18-20). Thats 776 gals a year. Times that by .20 (.10 if comparing mid and prem), the diff between reg and prem gas. That's $155/yr. Divide that by 12, (months). $13 bucks a month for me.
I know some of you drive a lot more miles than I do. Just plug in you own mileage numbers. The stuff in the middle is tha same, and the final # will vary.
For those of you that don't figure your miles, it's probably $1.50 to $3.00 more a TANK for prem., depends on how many gals you get and what grade your using now.