There seems to be a lot of variance in the percentage of police chases that involve stolen vehicles but the numbers seem to be in the range of 10% to 70% and even the low end of that isn't particularly low.
You also have the problem that if you steal a car and then run from the police the result is that they don't pursue you and send a ticket to the person you stole it from, that makes it a lot easier to steal a car, and then the percentage goes up.
Except that the person trying to get away knows that too, so if all they're doing is buying themselves a bigger fine, why are they doing it?
The answer to that could be because they stole the car, or because there's a body in the back, in which case mailing them a fine doesn't work.