Drivetrain loss is a common topic of conversation in the tuner world because any time you strap your car to a chassis dyno, the output being measured is at the wheel, not at the crank like the published SAE net horsepower figures used by the auto industry. Strap your 298-bhp RevUp G35 Coupe to the dyno and you may be disappointed to see little more than 220-230 horses measured at the rear wheels. Where did that 60-plus horsepower go missing? It was used up in a variety of ways before it could reach the drive wheels, the primary source being what's broadly described as drivetrain loss.
What's interesting about this example is that when you do the math you'll see that the percent loss is much higher than the 15 percent "rule" you'll find in any number of online threads on the subject. For whatever reason, drivetrain loss seems to be one of the most poorly understood subjects discussed on online car forums, so despite my love of the Internet and the limitless pornography it makes available to me, when it comes to a fairly technical subject like this it's hard to find good information.
A few years ago, I needed to educate myself on drivetrain losses while heading a rulebook committee for a local racing series that wanted to use dyno tests to measure engine output and then convert the results to net horsepower. After fruitlessly Googling and sifting through endless car forum threads polluted with half-truths and misinformation, I turned to the same source that developed the current manufacturer horsepower standard, the Society of Automobile Engineers (SAE). On its website you can access brief summaries of technical papers published by some of the world's leading automotive engineers and download the complete documents for a relatively small fee (usually less than $10 per article). As luck would have it, in 2002 the SAE held a symposium on transmission and driveline systems, and the papers that came out of it covered drivetrain loss in great detail.
One of the first things I learned from reading these papers was to completely disregard the 15 percent drivetrain loss "rule" (or any other percent value) that so often comes up during online discussions of whp versus net horsepower. The fact of the matter is every vehicle experiences different levels of drivetrain loss as determined by the design of its transmission and driveline components. Simply put, the amount of horsepower lost to the forces of inertia, drag, windage, pumping and friction are different for every engine, transmission and driveline design.
So the total power lost between combustion and forward motion is specific to each vehicle and therefore no single rule, percentage or fixed number, could possibly apply to all vehicles. Even on the most superficial level, this is easy enough to understand because an all-wheel-drive Subaru obviously has a lot more driveline components to spin (front, middle and rear differentials along with front and rear driveshafts and two prop shafts) and a beefier transmission to hold all that turbocharged torque, so it's naturally going to suffer from greater drivetrain losses than a Honda Fit with its much smaller and less robust transmission, smaller and lighter driveshafts (and no prop shaft) and single differential.
Breaking down the different types of losses that occur within a vehicle's drivetrain, steady-state losses occur while the vehicle is cruising at a steady or constant speed, where average angular acceleration is zero because no additional torque is being called upon to accelerate the drivetrain's rotational mass. Within the drivetrain, steady-state power losses occur from the following components: the transmission torque converter (in the case of automatic transmissions), the transmission oil pump, clutch pack drag, one-way clutch drag, seal and bearing drag, gear windage and friction, and final drive losses.