There is an interesting USA Patent for an improved method of estimating the aerodynamic coefficient of drag of a real vehicle out on roads.

Near the end of their description of their aero method, they also describe a neat way to better estimate transmission and differential friction and “stored rotational energy” in driveshafts and wheels.

They jack up and put the vehicle on jackstands, start the engine, engage the 1:1 gear and bring the drivetrain up to a speed. They measure the rpms of the wheels, then take the vehicle out of gear and let the drivetrain coast down to a lower rpm ( but not zero). After gathering this data, they remove the wheels, and re-run the coastdown again without the wheels.

With the two sets of data, they could calculate both drivetrain friction and stored rotational energy if they just knew how much rotational energy the wheel and tires have.

They devised a neat way of measuring this.

Think of Chrysler torsion rod suspension.

These inventors made a “torsional pendulem” by fixing a small diameter steel shaft to a ceiling and welding on a plate to the other lower end of the steel shaft with studs to match the bolt pattern of the wheels.

They attach a wheel/tire to the plate, then twist the tire on the hanging pendulem, and let it go.

The wheel/tire will twist back and forth in a cycle whose twists per minute depends only on the rotational inertia of the wheel/tire/mounting plate, plus the “spring rate” of the steel shaft. It is a easy to build “ I of a wheel” measuring instrument, where I is the symbol scientists use for “Rotation Inertia” of a rotating mass.

Old fashioned mechanical wrist watches have a similar torsional pendulem inside them.