How Do Bike Manufacturers Calculate Mileage?

For motorcycles? When I heard "bike", I think of one you pedal.

In the US, there is no standard for motorcycles, so manufacturers quote a figure that seems reasonable, often based upon testing, and sometimes based on testing that approximates the EPA (our Environmental Protection agency) test required for cars. The car tests are done on dynamometers under closely controlled conditions.

But for motorcycles, its whatever the company thinks they can get away with or (if they are more enlightened) a figure that will not leave their customers disappointed and angry. Several motorcycle manufacturers do not quote any mileage figure at all in their specifications.

calculating Mileage?

Assuming you mean MPG, even if they quote ... and even if you realized the advertised MPG, your actual experience will usually vary (on the down side).

Break-in operation (at only low speeds) is apt to match or exceed the advertised MPG; but the higher the speed the lower the MPG, so if you can get within about 15 MPG at highway speeds (versus break-in speeds), it's not a big deal.

If you are testing your own MPG when filling the tank, you should also take into account that MC's typically "measure" and record (on odometer) miles/kilometers which are somewhat to substantially shorter than actual miles/kilometers ... this apparently in order to accelerate depreciation/devaluation of "bikes" ... in order to encourage new sales, fudge (cheat) on warranties, and enhance financing (trade in) hypothecation in dealers' favor. So it's essential to determine the built-in error (design error ) of your bike's odometer in order to accurately determine actual MPG.

Standard procedure ... ? Yea, right.