Fewer BTU/gallon (does that mean "weaker"?), but not that many fewer.

Gasoline has 117,600 BTU/gallon
Ethanol has 67,000 BTU/gallon

E10 by definition is 10 % ethanol, 90 % gasoline

Thus:

E10 BTU would be (.1) times 67000 plus (.9) times 117,600
E10 BTU = 6700 + 105840
E10 BTI = 112540

Difference 117600 - 112540 = 5060 BTU/gallon fewer
112540/117600 95.7 percent

So E10 is 4.3 percent "weaker" than gasoline.

E15 or E85 figures may be calculated in the same manner.

Jon

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