Ethanol is closer to 50% less BTU by volume vs. gasoline. So, E10 will drop your MPG by about 5% compared with pure gasoline.
That said, my experimentation with E85 (Tundra is flex fuel) was interesting, and didn’t follow prediction. E85 should be a roughly 35% drop (Ethanol has 55% of the BTU by volume), but I saw about a 25-30% drop in MPG and the truck ran better.
Two theories:
1. E85 is “up to” 85% Ethanol - so the batch at the local NEX pumps may have had less than 85% ethanol
2. E85 is much higher octane and this may have allowed the engine to run better ignition timing
Didn’t put a scan tool on it to verify any of the theories, and the price of E85 rose enough that it no longer made sense. When E85 was $1.99/gallon and regular was $2.99, then, E85 was cheaper per mile. But it was selling for nearly the same price, and the economics no longer favored E85.
Further, because the truck is flex fuel, Toyota halved the oil change interval. 5,000 for normal use, 2,500 for severe service. Regular Tundras, with the same 5.7 V-8 are 10,000 normal use and 5,000 for severe service.
So, clearly, there are concerns about the effects of ethanol combustion byproducts on the oil.