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I have a gasoline-engine car that is four and half years old and has been driven for 60 000 kilometers. I have noticed that the fuel efficiency is not what it was when the car was new. However, I lived further away from the workplace and had a longer commute with lower speed limits when the car was new. So, my question is whether an old engine has worse fuel efficiency than a new one, or whether the the change in my fuel efficiency is due to a shorter commute with higher speed limits.

The car has been regularly serviced with oil changes at 15 000 kilometer intervals. The original oil was 0W-20 but I have noticed that in the annual service, 5W-30 oil has been installed. I have noticed that the engine sound when idle might have become somewhat louder than what it was when new, and I can barely hear very slight piston slap after cold start that fortunately becomes soon unhearable after the engine has slightly warmed. I know the engine has special piston coatings but am unsure whether they last for the whole lifetime of the engine.

At least it seems to me that here in Finland where the purchasing tax and annual tax of a car is proportional to its fuel consumption, it is the incentive of the manufacturer to do whatever possible to lower the apparent fuel consumption, even if the solution doesn't last for the whole lifetime of the car. After all, the fuel consumption tests are made for new cars.

  • 60k km isn't that much for modern vehicles. The impact you're observing in fuel consumption is likely due to other reasons – Zaid Dec 23 '15 at 22:19
  • Definitely go back to 0W-20 oil. There is a reason for this, to include bearing tolerances and fuel economy. By using the heavier oil, you are causing undue wear on the engine. – Pᴀᴜʟsᴛᴇʀ2 Dec 26 '15 at 17:34
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However, I lived further away from the workplace and had a longer commute with lower speed limits when the car was new. So, my question is whether an old engine has worse fuel efficiency than a new one, or whether the the change in my fuel efficiency is due to a shorter commute with higher speed limits.

A four and half year old car is not outdated in terms of efficiency. The difference is in the distance covered, route, and driving style. A long commute on a highway (no traffic lights) is much more efficient than a short commute on a route with traffic lights. You will also tend to accelerate harder on shorter commutes with traffic lights in order to keep up with traffic, change lanes, etc.

Go back to the factory recommended oil weight. That can impact your fuel economy (a little bit) but most importantly, it will impact lubrication at low temperatures.

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What kind of difference are your noticing?

There are thousands of factors that affect fuel consumption. 60k is not high milage and there is little chance the engine is preforming noticeably poorer than new.

5-30 oil is significantly heavier then 0-20. If your in cold climate this could have a minor effect. at 60k I assume you have likely gotten new tires. Tires play a huge role in fuel economy. Make sure you have sufficient pressure. If your not using the same tires as the OEM then fill the tires to the pressure listed on the side of the tire, not the door jamb.

A bad alignment can increase fuel consumption. Getting a full alignment might help. Global warming might have you running your A/C more. As can driving at different times of the day. Idling obviously increases your average consumption.

The noise when cold is likely lifters or rockers due to the heavy oil. They are dry for the first few minuets until oil starts flowing freely.

If your only seeing 1-2 MPG difference its likely normal enviromental issues. If you feel a noticable lack of power then have your engine looked at.

  • No noticeable lack of power. The consumption when new was 5L/100km, the consumption now is 6.3L/100km, so it is 10MPG difference when converted to US units. It is statistically significant increase, but then again it might be due to shorter trips with higher speeds. – juhist Dec 24 '15 at 16:14
  • Very high speed 100km/h or higher can lower economy. Hard acceleration onto highways and hard passing will lower your average. Gas quality makes a defference. Im not sure if fuel production is as tightly regulated there as it is here in the US. If your filling up at a different location and/or different brand poorer fuel quality can make pretty large effects. Your oil is likely a large factor as well. – JpaytonWPD Dec 24 '15 at 21:06
  • Are you calculating based on fill ups or going by a readout on the car? – JpaytonWPD Dec 24 '15 at 21:06
  • Both. When I first purchased the car, I calculated both on fill ups and by a readout on the car. Then, at some point of time I decided the readout on the car is accurate enough and stopped calculating based on fill ups. So, because I have fill up values from the early life of the car and they were consistent with the readouts, I'm confident the values are accurate. The max speed on my commute is now 100 km/h (used to be 80 km/h earlier), and I now have to accelerate quite hard on a motorway ramp that has a steep uphill. I don't tend to pass other cars. – juhist Dec 24 '15 at 21:10
  • How about altitude? Does your new drive have more or steeper hills? Try some fuel injector cleaner, new oil, and a different gas brand to start. Air filter might be helpful also. Overall it seems theres not much reason to believe there are any engine issues. Some general maintenance items may improve things. – JpaytonWPD Dec 24 '15 at 21:52
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It seems to me that it would perform worse over time since parts are going to be worn out.

From what I understand piston rings can become worn out and not seal the piston very well in the cylinder. If that happens then the combustion gasses can move past the rings and get into the inside of the engine. That would be wasted energy and would make your fuel efficiency worse.

  • While what you say about piston rings is true, how likely is it that they will have worn after 60k km? I'm not saying it's impossible, just that it's highly unlikely that this will explain the poor fuel economy – Zaid Dec 23 '15 at 22:22
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    However, I said that I can barely hear slight piston slap after cold start. This is a new phenomenon, which did not occur when the car was new. – juhist Dec 23 '15 at 22:23
  • That makes sense. I would think any wear would effect the efficiency at least a little bit, right? – Ppoggio Dec 23 '15 at 22:23
  • Is it even normal to hear piston slap at 60k km? Maybe you should consider that this is some other sound? or you have engine damage. The diference in sound can also be due to outside temperature and oil type. – Evren Yurtesen Mar 6 '17 at 10:19
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Observation over time (that is, our family having owned a number of vehicles for large parts of the vehicle lifetimes) is that as engines run in they tend to get a bit more efficient rather than less efficient... and then the efficiency drops as the engines continue to wear. We suppose it's because when manufactured, parts are tight and there's some friction losses within the engine components... as they wear they reach a sweet spot where the internal friction and leakage losses are a minimum and. From experience, VW 4 cylinder petrol engines seem to get better for way over 60,000km... in practice maybe three times that.

On that basis, I doubt that your engine wear is responsible for reduced economy. Change of driving pattern is far more likely to contribute to reduced economy as is poor maintenance and mis-inflated tyres. As example of poor maintenance... braking systems can get corroded and dirty and disk brakes can drag slightly... even when the manufacturer service specifications are tightly followed. Stripping, cleaning, replacing failing parts and re-greasing all slide components in the braking system can up the mpg (I've seen 2-5mpg improvements ... ymmv however)

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Moi Juhist,

If you are driving short distances you will use more fuel. Because your engine will use more fuel until it reaches to operating temperature. This makes a big difference in Finland due to cold winters. In addition the winter tires will have higher rolling resistance. Also the type of oil effects this, 0W-20 oils has lower cold and warm viscosity compared to 5W-30 therefore it will reduce fuel consumption. (thats why they sell 0W-XX as fuel saving motor oils). I think if you change 0W-20 with 5W-30 you will save few euros on the oil, but then pay more if you are driving a lot on fuel.

Let me give an example, I have a 2002 VW Golf MK4 with 260k kilometers, which has a highway consumption of 5.6l/100km according to handbook. When I drove from Turku to Helsinki in summer, very carefully, started with warm engine (resetted the meter when I started trip) not exceeding ~80km/h, with AC off and trying to break as little as possible, I managed 5.4l/100km (according to computer) with 0W-30 oil. My fuel usage result is better than what manual says. So, I would say the old/used engines does not consume more fuel or there is no significant difference.

Perhaps the difference is because 0w-30 did not exist when this car was produced and maybe VW considered AC to be on when getting the figures originally, so I could beat the specs of the car :)

When I checked the previous service receipts. I also realized that they use cheapest oil. So, last time because I don't trust the shop to put good oil (since they try to maximize profits) I went to motonet(name of a finnish autparts shop if anyone wonders) and bought 0w-30 and gave it to repair shop when the car was in oil change last time.

To sum it up, probably the problem is probably because you do not have exactly the same test conditions. I recommend that you put 0w-20 oil at next change, wait until summer, warm up the car, reset the meter and then drive ~100+km on highway with ~80km/h (I believe this is the speed that most cars have best efficiency) then check the result with your owners manual. If you match it or come close to it, then I would say everything is fine.

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