There's no single answer to the mileage of a 1989 Chevy G-20 van. Mileage depends entirely on how well the van was maintained, how it was driven, and its overall condition. A well-maintained van could have relatively low mileage (though "low" is relative for a vehicle of that age), while a neglected van might have very high mileage or even be non-functional.
You'd need to look at the specific van's odometer reading to know its mileage.
