I spent a lot of years riding dirt bikes out there in my youth. IIRC, the lake bed was not that smooth and in no way would I have wanted to do a top speed run in an automobile on it.
60-70MPH on a dirt bike was no problem though.
BTW, if you have ever seen the first Lethal Weapon movie, I believe the dry lake scene was filmed there. Gives you some idea of the surface.