That would be true in the US but depends on how the gasoline is rated over there: Motor, Research (how gasoline was rated before about 1975), or PON (Pump Octane Number or R+M/2) something the US uses to confuse the natives.
Good rule of thumb is PON is about 4 numbers below Research so 87 is about 91 old style, 89 is 93, and 93 is 97. This would make the old style 98 octane premium about 94 here today. 89 is an old style regular and 87 is just trash.
Octane has nothing to do with energy, just knock resistance so the most economical is the one the engine runs best on though for long Interstate trip with a light load in cool weather you can probably drop a grade. Just avoid flooring it. A higher grade than needed is just financing the oil companies and using more crude than you need to.
For more than you ever wanted to know, see here: http://en.wikipedia.org/wiki/Octane_rating