The recommended fuel as per the Owner's Guide is 87 octane. You can use premium if you want top performance or tow a lot. 87 octane in my vehicle provides plenty of power. I don't see a need for premium whic by the way will cost you more that $2 more. Right now the average price for regular is $1.379 a gallon and premium is $1.539. On an 18 gallon (Imperial) fill up that is a difference of $12.96.
Peter
I wrote this up on another forum a few weeks ago, I'm using a custom tune from Livernois, so I don't know how closely this will match a stock tune, but I assume the results would be similar.
When less expensive gas isn't less expensive...
Loaded up a tune for 87 octane gas last week because gas prices hit 4.33 for regular here. Figured I'd save a few bucks per tank this way.. decided to do the math this weekend on my savings.
I saved ~.20/gallon by switching from premium. I have an 18 gallon tank so that's ~3.60 saved for the tank. Awesome.. at 2 tanks a week that's a lunch!.
Then comes the messed up part of the math.
Efficiency lost during daily driving by having boost and timing tuned for 87 octane is roughly 1.2-1.4mpg. I assume it requires a lot more gas to do things like enter the freeway (get up to speed), speed up to pass cars, etc.
1.2-1.4mpg is roughly 21 miles per tank lost to having less power.
21 miles is almost exactly what I get per gallon on average
so.. loss per tank is ~4.30
So in conclusion my net gain by switching to 87 octane was a loss of around 70 cents.
If the engine is optimized for higher octane, use higher octane. Even Ford's brochure showing HP, Torque, and EPA Ratings says all values were obtained on 93 octane gas.