Thank you all for your really helpful advice!

Midwest Michael, thanks for pinpointing those problems. You 're absolutely right on the "breathe" matter and, i must admit, it was an unsuccesful expression coming from a bad translation. Certainly our goal is to cover distance through scenery more than staying on a place, mixing with the locals and really feeling the place.
Nevertheless, we'll probably give another day to Yellowstone since it is a must see and all of you think it requires it. In any case we have a schedule that allows a four-day delay of our planning exactly for that purpose. So, the coastal road from SF to LA will take the extra day it deserves, as well.
The reason we don't rent in Toronto is because renting a car there was more expensive and we couldn't get the insurance the ALI coverage that the US renting was offering. Renting in Toronto also includes a mileage fee (0.12/mile over 2600miles) and border crossing was an issue as well. In any case we'll have to drive back to Buffalo and maybe pop to Montreal through Toronto...

The car is not a Chrysler (sorry!) but a Chevrolet Cruze. It is categorized as "Intermediate". Would it be ok or should we jump up to "Standard"?

edmar434 and AZBuck, thanks for the optional routes and the suggested visiting places. Thank you all for the gas estimation (really helpfull!!!) and I'll surely try to keep you posted while doing it!