I've done the testing over fifteen full tanks of gas (3 separate tests @ 5 tanks/test) calculating tank average fuel economy. From a fuel economy stand-point only, you save money in fuel economy by running premium if and only if the difference in price between regular and premium is $0.40/gallon or LESS. If the difference goes above about $0.40 you begin breaking even.
Test Disclaimers on 04 GXP:
1) Mileage = 26-29k
2) Driving conditions - town and highway combined average
3) topography - hilly, few flat spots, except test 3 which was done on a trip to Michigan, i.e. mostly flat driving after Pittsburgh.
4) "normal" regular and premium gas from same station(not "special" gas from Exxon/Mobil or BP) except test 3 which was also "normal" gas but not from same station as tests 1 and 2.
5) all tested in spring, summer, fall conditions with no A/C and no extra load on the electrical system beyond OEM equipment, windows down.
6) car driven only moderately hard, only a few hard starts
7) new tires beginning test 1
8. test 1 and 3 had oil change within 500miles of beginning of test.
9) no fuel economy "improvement" devices
10) color = crimson tintcoat
Worst average fuel economy from one tank
= 17mpg nearly exclusively town driving in WV and Pittsburgh (a little interstate between my house and Jerry'* in Squirrel Hill)
Best average fuel economy from one tank
= 29mpg - fill up on OH turnpike at Eastgate measured at end of tank just south of Lansing, MI around Jackson.
I was surprised at the variation (2 standard deviations) between the best and worst mileage (although an n=15 doesn't really allow for a reliable measure of SD). From a non-statistical point of view this was unexpected. I would guess it would narrow as the car gets older. If I were to choose the single outlier it would be the 29mpg tank. Most measures came in between 21.5 and 24.5 mpg.
Again, it pays to run premium from a fuel economy/cost consideration IF the difference in price between reg and prem. is LESS than $0.40/gallon.
I haven't seen testing that shows premium is better for the engine mechanically
so can't comment on that aspect.