So much confusion on this thread!
There are four ethanol/gasoline blends available in Thailand:
E85 = 85% Ethanol, 15% Gasoline
E20 = 20% Ethanol, 80% Gasoline
E10 = 10% Ethanol, 90% Gasoline
Gasoline = 0% Ethanol, 100% Gasoline
E85 is cheapest and contains least energy. Gasoline is most expensive and not commonly available.
The ethanol content also effects the RON rating (octane content). For simplification we'll say E20 and E85 are only available in RON95. Most of what is sold is E10 and advertised as 91 or 95. These numbers refer to the octane content (note - not percentages, in other countries 101 and 103 (and others) are available). I'm not sure why they don't phase out 91 completely as the price is virtually the same as 95 and anything that can run on 91 can run on 95 (with vice-versa not true, my '04 Ford Laser only runs on 95).
Diesel will be in the news shortly with them increasing the biofuel content from B7 to B10 - 7% to 10%.