I stole this from somewhere else - but it's similar to calculations I did a year ago (and ignore their moronic schmaltz about BTUs being the only real energy measurements)-

"What is the real energy content of E85? How many btus in a gallon of E85 in comparison to unleaded gasoline? For that matter, what is a btu? How far can a vehicle operate on E85 in comparison to gasoline? The first step in answering these commonly asked questions is to determine a common denominator.

There is only one common unit of measurement in reference to energy, the British Thermal Unit (btu). This is a standard unit for measuring heat energy. One btu represents the amount of heat required to raise one pound of water one degree Fahrenheit at sea level. More commonly, one btu is the energy produced by a typical wooden kitchen match.

According to the American Automobile Manufacturers Association, a gallon of typical gasoline contains 114,132 btu’s. However, even this amount of energy content changes from summer to winter as gasoline’s volatility is seasonally adjusted. For the purposes of this summary, we assume the following:

1 U.S. Gallon of gasoline contains 114,132 btu
1 U.S. Gallon of no. 2 diesel fuel contains 138,000 btu
1 U.S. Gallon of ethanol contains 76,000 btu
1 U.S. Gallon of methanol contains 56,800 btu
1 U.S. Gallon of propane contains 84,500 btu
1 U.S. Gallon of compressed natural gas contains 19,800 btu

Ethanol is denatured by adding 5 gallons of gasoline to 100 gallons of ethanol (4.76%), therefore:

Ethanol @ 76,000 btu/gal x 95.24% = 72,382
Gasoline @ 114,132 btu/gal x 4.76 = 5,433

Denatured ethanol = 77,815 btu/gallon

E85 is then a blend of denatured ethanol and gasoline, therefore:

Denatured ethanol @ 77,815 btu/gal x 85% = 66,143
Gasoline @114,132 btu/gal x 15% = 17,120

E85 = 83,263 btu/gal"
E85 website
Which works out to about 73% of the energy contained in gasoline - not that your fuel efficiency will drop to exactly 73% (hard to say whether engine would be more or less efficient with more etoh - I go 20-22mpg around town in Iowa with 90/10 gas/etoh mix on my 92 244 auto), but a 10 - 20% drop wouldn't be unexpected.

Either way - if you do try an E85 run - I'm curious to hear about the results!

Good luck,
Zach Zaletel
Anchorage, AK
1990 245 w/238k mi