Fuel consumption in a brand new car vs. old(er)

Discussion in 'Community Discussion' started by sammich, Jun 21, 2009.

  1. sammich macrumors 601

    sammich

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    Sep 26, 2006
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    Sarcasmville.
    #1
    I read somewhere (a long time ago) that extremely new cars would have a higher than rated fuel consumption. I don't see the logic in that statement now but I guess I still want someone to refute it.

    This was brought about recently because our BMW 320d was acting up, got fixed, went to pick it up, drove 5 metres and the engine cuts out. So we asked for a different interim car (was a 120d, changed to a 323i). So this 323i is a brand spanking new one, it was a service car and had 120km on the odo.

    So I'm driving this thing along in reasonable traffic in Sydney, and I'm having serious trouble getting the average consumption down from 17L/100 (13.84 mpg). The traffic did ease a bit and I was at 16.3 by the end of a 25 minute trip. (for comparison, in the same traffic our 320d would probably get 7L/100km or 33.6mpg).

    I mean, there is absolutely no reason that a 2.3L engine (or whatever size it is) can use that much fuel for commuter driving. Would the fuel usage go down as the car builds up a few km?
     
  2. ElectroGhandi macrumors regular

    ElectroGhandi

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    Jan 9, 2009
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    Baltimore, MD
    #2
    Was the loaner car an automatic transmission and your car a manual? Was there a computer in the car calculating the MPG? It could have been off. I bet if you drove it more and in a more varied environment, it would have gone way up. The 3 series is a small car, there is no way it should be getting 16 MPH. My Isuzu Rodeo SUV gets about 20 mpg.
     
  3. Osarkon macrumors 68020

    Osarkon

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    Aug 30, 2006
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    Wales
    #3
    I think it does depend more on the car itself than the age.

    For example, my current car, which is a '99 Peugeot 306 1.9d, gets 44mpg.

    My friend's '03 Vauxhall Astra gets 30mpg on a good day. I think it's a 2.0 turbo.
     
  4. Rodimus Prime macrumors G4

    Rodimus Prime

    Joined:
    Oct 9, 2006
    #4
    A brand new car has a brand new engine. The new car you are driving only have 120 KM (75 miles) on it total. Until the engine is broken in at around 4800 km (3000 miles) it going to burn a little more fuel.
     
  5. sammich thread starter macrumors 601

    sammich

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    #5
    (I just realised that there might be some confusion over the units. Unless explicitly mentioned, I'm talking in Litres/100 km.)

    Had to give back the car now, back with the 'ol diesel.

    All the cars are autos, the average reading came from the dash and there was the analogue/needle instant fuel usage too. I've driven it more now, it has gone up to about 16mpg, but still rather unacceptable.

    I know all about diesels vs. petrol cars, but it honestly shocked me to see how much more fuel in particular instances (like moving from a red light, cruising or inclined roads) that petrol unit used. Speeding up slightly going up a mild incline uses barely 10L/100km, but the 323i was 20+. But then again NA petrol engines are far more fun to drive :D

    This is what I suspected, but is there some reason behind this? Wouldn't logic tell you that the more a component is used the less efficient it is?
     
  6. Abstract macrumors Penryn

    Abstract

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    #6
    And funny thing is that the engine will probably burn more fuel when its older, where its worn in enough to not be too stiff (it's all mechanical parts, after all), but also when its not worn out and not as efficient. ;) Cars have a peak, and it doesn't come when its new.

    And your problem isn't that the car is new. Your problem is that the car's computer is off. It's an estimate anyway. It happens on my parents' semi-new car when the fuel tank is too full. After driving 50 km or so, the reading becomes more realistic.
     
  7. Vogue Harper macrumors 6502

    Vogue Harper

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    #7
    It depends on how the computer calculates the average mpg. For an accurate reading it probably needs more than 120km on the clock because it is not inconceivable that most of those was spent in traffic. Also, as it was a service courtesy car, most people would not have driven it with economy in mind and would have been using kick-down quite a lot.

    The reason for the different performance you noticed between diesel and petrol engines on e.g. pulling away and driving up inclines is because diesel engines by design have better low range power, they generate more torque - with petrol engines you need much higher revs to generate the equivalent amount of torque.
     
  8. mkrishnan Moderator emeritus

    mkrishnan

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    Jan 9, 2004
    Location:
    Grand Rapids, MI, USA
    #8
    When you posted mpg, are those miles per imperial gallon (4.54 l) or US gallon (3.78 l)? And is it the same US mile (1.61 km)? Those numbers seem quite bad for a 323i... The 328i is the smallest engine variant sold in the US and its city mileage rating is 18 (about 13l/100km).

    If the gages seem right (I wouldn't be surprised if the estimated economy is way off), then there might be something wrong with this loaner also....
     
  9. sammich thread starter macrumors 601

    sammich

    Joined:
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    Sarcasmville.
    #9
    That's one of the things I love about a good diesel, it doesn't muck around wondering if it should change down a gear because it's suddenly hit a hill. It'll charge up it at 1k revs if it has to.

    SEE?! This is why the world needs to change to metric. It just makes sense. But that's a discussion in another thread. Anyway, I used the US mpg (hopefully) in those numbers.

    Maybe it was an anomaly/new car thing, and needed some running before it got a good enough reading. I just wanted some assurance because if I were to buy looking to buy something like that I wouldn't be staring that kind of fuel usage day in, day out.
     
  10. mkrishnan Moderator emeritus

    mkrishnan

    Joined:
    Jan 9, 2004
    Location:
    Grand Rapids, MI, USA
    #10
    Unless you drive like an insane person or keep the parking brake engaged, it seems unlikely to me. Even the aggressive drivers at American test magazines usually do at least the City (American) EPA rating in combined driving... the city rating is supposed to be a lower bound on what one might expect.

    I haven't owned a new BMW before, but with other cars that I owned right from day one, there was not particularly lower mileage during a burn in period. I mean, I might get 34 MPG instead of 36MPG, but definitely not 24 MPG instead of 36 MPG or such a large difference as you describe.
     

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