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I have never heard of this, and in Google searching for this, I did not find anything on this.

However, it seems reasonable that in the below scenarios, in measuring the condition of a used engine engine (a) gets much worse than it's stated fuel economy engine (b) gets exactly its rated fuel economy

It would be reasonable to hypothesize that engine a is not as in good of condition as engine b.

Here is my question: How much value would you place on fuel economy as a measure of an engine's condition after a used lifespan? If a 15-year-old engine gets its rated fuel economy in the vehicle, is it likely in good condition? If it gets much worse fuel economy, is it likely in bad condition? How much information can be had from looking at fuel economy?

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    Fuel economy is very difficult to measure. It varies heavily depending on the driver, and that margin is usually bigger than the worn engine effect. Bottom line: you'll never know if the particular number you get comes from the driver's habits or engine condition. – Agent_L Aug 30 '16 at 12:27
  • Well I respectfully disagree, assuming I can accurately measure how much fuel is used - I can drive the same make and model automobile the same route with the same driving habits. This is reasonable for someone looking at used cars that offers to meet at the same public place for a viewing and test drive. – Drawninpictures Aug 31 '16 at 2:07
  • @Drawnpictures I think we do agree. My point was about comparing numbers coming from different sources: from the seller and "rated" mileage from manufacturer (which both will be lied about on top of any errors). Both of them are hard to compare to your driving style results. But if you can drive the car in question and an exactly same model in perfect shape as a reference then it should work as an indicator of engine + drivetrain condition. – Agent_L Aug 31 '16 at 9:36
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There are a large number of factors that contribute to fuel economy. As others have mentioned, driver style, driving condition (hwy/city), and vehicle condition/weight (roof rack, towing, loaded full of kids/gear) will all have an effect on fuel economy.

However, to answer your question, we will assume an unloaded car with a mixed duty cycle and a reasonable driver (some WOT, but usually moderate throttle driving). You cannot immediately assume one engine is in worse shape than another, because there are several other factors at play.

One major factor that is easy to check is the state and condition of the tires. Under-inflated, mismatched, or balding tires can cause significantly more drag than new tires. Ill fitting body panels will also cause more aerodynamic drag, but less obviously, missing panels under the engine and rear axle of the car can cause significant drag in a much less visible area.

An errant o2 sensor or spark plug could also be to blame for the poor economy, and such consumable items I would not necessarily classify as representative of the state of the engine. Old or dirty transmission or engine oils could also be causing poor vehicle performance, the latter would cause significant wear on the engine, but the former would only cause the engine to work harder.

All things considered, high fuel consumption almost certainly indicates there is something 'wrong' with the car, but you cannot immediately pinpoint the engine as the culprit.

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Technically, yes. As engines get older, they do get less good fuel economy, and when they are totally made over, their fuel economy improves.

However, generally the time you're trying to determine the condition of an engine in a general way is when you're buying. And when you're buying, you can't necessarily get an accurate fuel economy rating from the original owner, as you don't know how his/her driving habits and driving conditions (city, highway, pulling, a mix of the above, etc.)

Therefore, generally, when buying a vehicle, you ask mileage, oil changes, repairs, and other concrete information. On the other hand, when it's your vehicle, you can make better guesses by looking at the spark plugs, running compression tests, etc.

This is probably why you don't find any information on Google.

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