Speedometer off by two mph

That is well withing the tolerance of tire sizes specified for that van. You can take ten different tires with the same sizes on them and when installed and inflated they will measure slightly different in actual diameter depending on the tread style, depth, tire construction and rubber compounds. Even something like changing the inflation pressure by a few pounds will alter the actual ground speed.

Thirteen mpg is about normal for a van of that time. They are a work truck so the gearing and transmission are designed for weight not mileage.