What is the difference between E100 and E85 in fuel efficiency?

If E100 is [near] pure ethanol, how does that affect its efficiency compared to E85 and normal unleaded? How much difference would it make under the most recent and technologically advanced engines if you used E100, E85 or unleaded petrol? And, is it true that E85 will actually give you more distance than E100 in a specially designed engine for each?