UPDATED- (Speed question.) Well, I finally sat down to figure out mileage on my cross-country trip from Chicago to Arizona and back. We took the southern route (Kansas/Texas/NewMexico) down, and the northern route (Utah/Colorado/Nebraska) back. And...the verdict is...it ranged between 32 and 43 miles per gallon -- pretty much the same as on the flatlands of the midwest prairies, averaging 38 mpg with the air-conditioning running pretty much full blast. This was driving mostly at 70-75+ mph, and mostly automatic, except switching to manual on the fly to handle big hills and mountain climbs. This is also driving with a full load -- two zoftig adults and a week's worth of luggage crammed in the back.
For all you asking about downshifting on mountain roads--I definitely recommend it! The car will downshift on its own if you punch on the gas going uphill, but the car is peppier and more responsive if you switch to manual and downshift earlier--especially if you are trying to pass another car or truck--you definitely don't want to lose momentum then.
Anyhow--I've gotten pretty good at manual shifting now, though in city driving I don't use it much. And, the scenery through Arizona, Utah, and Colorado is magnificent--I recommend a road trip to everyone, and you will leave smart fans in your wake! There's a group of teen-agers in Nebraska drooling after the car, a motorcycle mama in Missouri who said -- "Only two seats--no room for the kids--I want one!", and lots of truckers saying they'd be handy to have to get around town in.
My last tip, if you buy a bottle of that fabulous "Prickly Pear" Vodka in Flagstaff--it was bottled at 7000 feet, and will pop its seal as you go to lower altitudes...so keep it standing upright, in plastic bags, and consider cracking it open early.
Thankfully, the car was very comfortable on the long haul (and I have a bad back). The only downside to my trip was the rock that jumped up to crack my windshield near Prescott, AZ, which cost a bundle to replace once I got back to Chicago, but the team at Lincolnwood took care of it expertly and we're good as new!
Happy Trails to all.