You cannot run E85 in a car that isn’t set up for it because of differences in the programming required for fuel injection. E85 and gasoline have different stoichiometric ratios, roughly 14.7 by weight for gasoline, 9.0 for ethanol. In plain language, to achieve ideal combustion, more E85 must be injected into the mix than gasoline, when you’re running on it. The computer must adjust the injector pulse width to produce the correct air/fuel ratio for each fuel. In a vehicle that’s not designed to burn E85, the mixture would be wrong.
There are also some physical requirements in engine and fuel system design that must be considered. Alcohol will damage some substances, like rubber, that gasoline won’t. However, almost any car on the road could be modified to use E85 by retrofitting the affected components with ones that are compatible with both ethanol and gasoline. This has been done for years in cars that have been modified for racing.