Can Sonic beat Flash in a race

The only way to answer a (pointless) question like this is to consider their abilities within their respective canons.
Flash is routinely shown as being capable of exceeding the speed of light by a relatively large factor in his comics.

Sonic is... much, much slower than that, at least in the games. I don't know how fast he's claimed to be in his comic books.

I suspect that in a Sonic comic, Sonic would win, and in a Flash comic, Flash would win.