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On my Eurovan, gas mileage dropped from an average of 25 mpg to an average of 22 mpg, so that's a bit more than 10%. Back in the '90s, before we had E10 all the time - only "oxygenated" gas during the Winter months - my Audis (5000S and 80 Quattro) would regularly drop 2-3 mpg when the supply was switched the "Winter blend". That was right around 10% for each, so my experience with fuel injected, knock sensor equipped automobile engines seems to agree with Lannis' experience. The same can not be said for any of my carbureted, air-cooled, "low-tech" motorcycles.
If it did, then running straight ethanol ought to yield 0 mpg!
I always surprised and wonder why some report zero effects with E10 on older equipment. Having spent several years associated with a M/C shop that also dealt with small lawn equipment, snowmobiles, and such how busy the mechanics were every spring (or fall, depending on the equipment) fixing issues specific to ethanol fuel. We would, on a regular basis, see plenty of carb issues, fuel line issues, tank corrosion, and the occasional other issue.