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I realize fuel efficiency is not just about the engine, but the engine is surely a key factor so I hope this question is accepted here:

10~11 years ago if someone wanted a fuel efficient car in a certain class, they would have landed on options like the Toyota Prius or Lexus CT. (they have a second engine but only consume gasoline, meaning their efficiency is simply distance/liter).

I was looking into new car options today, figuring that a decade is a fair amount of time to find improvements, and was really surprised that most new cars do not even advertise the efficiency that these cars promoted 10 years ago.

I considered that perhaps definitions got stricter, so instead of looking at advertised efficiency I decided to compare the actual efficiency of these 2011&2012 cars, to the actual efficieny of several other cars via spiritmonitor.de

I just checked a few cars that came to mind, but in the end pretty much any car appears to be less efficient today than these models were 10 years ago. Even the Lexus UX cannot keep up.

Hence my question: Have gasoline engines (or cars) really not gotten more fuel efficient in over a decade?

If so, why?! Or am I looking at it in the wrong way. (For instance, the engines got more efficient, but safety regulations or other requirements led to less efficient cars overall).

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    Are you comparing like for like? The modern versions are undoubtedly more efficient and have a greater power output for the same fuel consumption. For example, a 1400cc car I recently sold had about the same fuel consumption as a much older 1400cc car I once owned (about 42mpg all round). But this one was 150PS power output, almost twice that of the old car. Commented Nov 7, 2022 at 22:53
  • @WeatherVane Is that really a good comparison, since the economy figures won't be done at maximum power output? You may well find that a more modern car uses far more fuel at maximum power output. That energy has to come from somewhere.
    – HandyHowie
    Commented Nov 8, 2022 at 7:48
  • @HandyHowie but we don't drive around using maximum power. Today, say a 1-litre car has a similar performance to an older 1.5 litre car, with better fuel economy. And at the same maximum power output, a modern car will use less fuel. Commented Nov 8, 2022 at 11:51
  • @WeatherVane Understood.
    – HandyHowie
    Commented Nov 8, 2022 at 12:55
  • Probably vehicle weights have increased along with efficiency, plus @WeatherVane's point that modern drivers expect more power, for a net effect of zero. The prius gained about 10% of its weight from the first gen to now. Commented Nov 8, 2022 at 17:09

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Here in the States, vehicle have to have been getting better over the last 10 years. According to Title 49 Subtitle B Chapter V Part 531 § 531.5(d), Table 4, the "Minimum Fuel Economy Standards for Domestically Manufactured Passenger Automobiles" for vehicles sold here in the states are as follows:

Model year    Minimum standard 

2011           27.8     
2012           30.7
2013           31.4 
2014           32.1 
2015           33.3 
2016           34.7 
2017           36.7 
2018           38.0 
2019           39.4 
2020           40.9 
2021           39.9 
2022           40.6 
2023           41.1 
2024           44.3 
2025           48.1 
2026           53.5

By statute, auto manufacturers have to improve their overall mileage for their fleets. If they don't, they face fines. You'll also notice between 2023 and 2026 (four model years difference), there's a HUGE amount of mileage gains which must be accomplished. Mind you, this is averaged throughout their fleets, but is still something which must be met.

One of the ways vehicles have improved in their mileage over the past 10 years is through the use of direct injection (DI). Speaking strictly about GM vehicles (because this is what I know best - I assume other manufacturers probably followed suit), the use of DI has propagated substantially is is pretty much the standard today. It was just starting to be used more main stream in the 2011-2012 time frame. DI does two things for the engine: It allows for better fuel economy as well as better power. This is just one way vehicles are doing better, but I'm sure there are other things as well.

Needless to say, I'd say your premise of vehicles not achieving better gas mileage cannot be proven. In fact what the government dictates the manufacturers have to be doing would show something else quite different is happening.

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  • Well the site I refer to lists significant measurements of actuals per model per build year. That being said I did not zoom in on any American brands/models (or in general on any cars that did not have the best efficiency 10 years ago), but will definitely look into this as it may indeed confirm the trend! Commented Dec 22, 2022 at 11:09
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It seems that the reason cars have not become more fuel efficient since 2012 is that car makers have optimized towards other objectives.

As mentioned in the comments, there may have been changes that increase the weight of the car (e.g. For safety or comfort). Or perhaps the power available to the driver.

I also found a claim that cars are now more optimized for passing tests with good CO2 scores, rather than real world fuel efficiency. It is of course hard to validate, but would not surprise me given the trend in regulations around CO2.

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    Similar to US schools where teaching is now optimized around getting kids to pass tests, not learn things.
    – FreeMan
    Commented Nov 16, 2022 at 19:36

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