Using the 85-percent gasoline mix in older cars, lawnmowers or boats is prohibited by law because the fuel may cause damage to vehicle systems. That's pretty rough, but according to Consumer Reports, that's still not enough for many car companies. According to the institute, nine automakers thus far have made it clear that they won't honor warranties of older vehicles found to be running E15 through their systems. Toyota, General Motors and Chrysler are all among those manufacturers.
There are other hurdles to E15 acceptance, too. If fuel stations need to provide E10 for older vehicles and E15 for those built after 2001, they'll need an additional storage tank for the new fuel. That can be prohibitively expensive, which means that it may simply be easier for older stations to skip E15 all together.