I fear that this question might be bordering on opinion based so sorry about that if it is.
When looking for cars one of the primary indicators that people seem to use when trying to determine the condition of the vehicle is the distance it has travelled.
It appears to me that this only gives limited information about certain components.
A large amount of the components which wear over time in a car spin, in some cases there is a linear relationship (or near enough) between how many times something has spun and the distance the car has travelled, for example wheel bearings.
However there are some components where there is not a constant linear relationship between revolutions and distance the car has travelled, the biggest example I can think of here being the engine. Due to gearing the speed of the engine will not maintain a constant linear relationship with the distance the car has travelled. An engine which has done 100k miles of motorway driving is going to have spun a lot less than an engine which has done 100k miles of urban driving.
Kinetic energy is proportional to the square of the speed, if the engine spins 100 times at 1000rpm the energy involved is less than if it spins 100 times at 2000rpm. High school physics will tell you that the factor here isn't 2 but is in fact 4. So we have identified a flaw with just measuring the number of revolutions, it depends on how quickly something is spinning. Measuring the work done (by effectively integrating the energy usage over time) seems to me like it would be a good indicator of how used a component is. The one advantage I can think of here is that taking a part well outside it's operating range (for example by spinning it much faster than it was designed to be) can significantly decrease it's working life but would not have a significant impact on this metric.
I guess I should probably actually ask some questions as part of this post:
- Why do we use distance as one of the main indicators to signify the condition of a vehicle? Should we be using any other information in conjunction with this when trying to determine the condition of components on the car?
- What other metrics are commonly used to indicate wear of certain components?