It's not good to generalize: Avis charges one-way mileage fees! Or, Hertz now charges one-way mileage fees! It is usually a provision of your corporate contract, which does not apply to the average retail renter.

The daily retail rates with unlimited miles are much higher than roundtrip. A mileage charge on a flat rate corporate plan may be cheaper for short distance, long time rentals.

There is a bug in the Avis computer system regarding pay now rates for one-ways. The total can actually be higher, because a separate taxable "one-way fee" is often added.