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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.

Revolutions

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.

Energy

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.

Questions

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?
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    for comparison, aircraft engines (which are sometimes just automobile engines) typically use hours instead of flight miles or revs – costrom Mar 29 '16 at 23:17
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    Great question. I'd also be curious to learn when cars started using odometers, and if it's just a case of "cuz that's the way we've always done it", since it was the best technology available at the time, and has since become the standard. – MooseLucifer Mar 29 '16 at 23:28
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    I think the reason has to do with giving an indication of complete car wear. Hours will only show how much an engine is used or ran. Mileage will tell you how much the differential, wheel bearings, transmission, etc., are used, as well as the engine. All it is is an indicator. Not as exacting as hours on the engine, but good for overall wear and tare on a vehicle. This is opinionated supposition, not fact, so leaving it as a comment. – Pᴀᴜʟsᴛᴇʀ2 Mar 30 '16 at 0:12
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    @costrom Can you name an aircraft driven by a car engine? I've never heard of that, and so much is different about how they are made I would love to dig into that more. – cdunn Mar 30 '16 at 3:32
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    @cdunn I can name a motorcycle engine that an aircraft uses. The Honda GL1200 and the GL1500 are both used by aircraft manufacturers. – DucatiKiller Mar 30 '16 at 5:01
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+50

This is an excellent question, and absolutely correct that there is no linear relationship between the miles driven and the amount of wear on the engine.

For machines which don't move much, or which move forward/reverse/idle in equal amounts, an odometer doesn't offer much additional information about how the machine was used. Thus these types of machines are fitted with an hour meter which gives a clearer representation of usage (although still not the full picture).

However, with a standard passenger automobile, it's assumed that most of the vehicle's usage involves moving in a forward direction, with negligible reverse and idle states. (Note that some odometers even count up when moving in reverse.) Therefore, an odometer reflects typical usage of a passenger automobile reasonably well, although extreme cases and usage (cold starts, long idles, many miles driven in reverse, highway vs urban miles) may not be reflected.

As you say, an odometer reading is one of the primary factors in deciding a vehicle's condition. This indicates that the odometer reading becomes more useful when considered in the context of other primary factors.

For example, you mentioned that highway miles effect less wear on the engine than urban miles. Thus, comparing the mileage driven with the year of manufacture can give a clue whether the vehicle was mainly driven on highways or in an urban setting. Conversely, hours of operation or total kW output of the engine may fail to yield this clue.

Also, odometer readings do give reasonable wear information about the parts which have a linear relationship to motion. Tires, wheel bearings, drive train, etc share this approximate linear relationship.

In contrast, relying solely on odometer readings is not a good indicator of wear for some parts. For example, basing engine maintenance schedules only on odometer readings will yield less accurate results due to the absence of usage context, and for this reason newer ECUs rely less on the odometer and more on driving habits and usage to trigger maintenance reminders.

So to summarize, odometer readings are helpful directly to indicate wear in some areas, and can be used along with other primary factors to yield clues about non-linear wear, thus improving the accuracy of a broad array of wear factors.

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