Portable dynamometer (wheel dyno, road dyno) vs chassis dynamometer

There are various solutions and devices available at the market for the performance measurement of motorized vehicles (cars, trucks, bikes, agricultural machines, etc.). People interested in buying dynamometer or dynamometer alternatives may be confused. Is that a good idea to spend more on a regular chassis dynamometer? Is the so-called "road dyno" or "wheel dynamometer" (a device connected to one of the car wheels) a good equivalent for professional power and torque measurement tool?
To be a devil's advocate, I will list the advantages of portable dynamometers ("wheel dynos" ):
- cheap (maybe not all, because some well-marketed gadgets may be pretty expensive)
- portable. You need a roll of double-sided adhesive tape, and you can stick such "dyno" to the car's wheel and voila! - you can start driving
There are no more real advantages.
The main disadvantages of "road dyno", aka "wheel dyno", are:
- wheel dynamometer does not measure true power and torque, but it's equivalent – because to measure power and torque by driving, you MUST know the precise weight of the car, front surface size, and Cx (air drag coefficient, specific to the model of car).
- you must drive the customer's car, accelerating at high gear (min. 3rd gear). If your company is in the town, police call "speeding" or "exceeding the speed limit".
- road dyno is cheap until the first accident with customers' car, especially with 100km/h+ speed in the city. Then – it is expensive.
- repeatability of results increases when the weight of the vehicle increases. Results for small, light cars are more inaccurate because the influence of other parameters increases with decreasing car's weight.
Unsolvable – where to find accurate data for any vehicle?- wind falsifies results. Also, air drag depends on Cx of the car and its front surface. How to switch off the wind?
Theoretically, be corrected by driving at the same time back the same road, but wind drag will increase incorrectness.- inexact flatness of the surface falsifies results. So unsolvable.
- wheel friction depends on the surface and is not constant.
It seems that to measure precisely. You need to know the exact vehicle weight with a driver, not too much fuel between a moment of weighing and measuring (as weight changes), use extremely flat roads, no wind, and wheels that do not change resistance much. Also, if your vehicle is heavy – results will be more repeatable.
Conclusion: wheel dyno is perfect for trains.
By the way – imagine that a patient is coming to cardiologist, reporting problems with the cardiac output. And the doctor says: "I have only a stethoscope instead of a real ECG (electrocardiograph) because stethoscope is portable. And much cheaper."
 For sure patient will seek another professional cardiologist with pro tools and devices.