Regular or high test gasoline

High test gasoline is for high compression engines that "ping" or predetonate when using low octane gas. "High test" or premium has more octane which slows down the rate of the explosion. Actually it has a little less power produced, not that you would notice it, but it won't help millage.
That said gas with ethanol has less power also. In that case the premium (if it dosen't have ethanol) could result in better milage.
Is it worth the extra money? Not usually.