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    OK hear is the question . Dose atmospheric conditions affect fuel consumption  ? Most of my driving these days is shortish runs on single carriage ways so according to the cars trip recorder the average MPG is 42 . I have just driven down to Cornwall and back so as a matter of interest I set the trip recorder to see what the MPG was on a good run , 228 miles each way  , same roads same average speed  give or take a mile or two per hour yet the difference between going 54.5 mpg and coming back 51.5 mpg . Not a lot I grant but going was dry and coming back was damp an wet so hence the question .

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    46 minutes ago, Gunman said:

    OK hear is the question . Dose atmospheric conditions affect fuel consumption  ? Most of my driving these days is shortish runs on single carriage ways so according to the cars trip recorder the average MPG is 42 . I have just driven down to Cornwall and back so as a matter of interest I set the trip recorder to see what the MPG was on a good run , 228 miles each way  , same roads same average speed  give or take a mile or two per hour yet the difference between going 54.5 mpg and coming back 51.5 mpg . Not a lot I grant but going was dry and coming back was damp an wet so hence the question .

    Don't forget it's uphill on the way home!:good:

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    Unless you were doing the test on purely scientic basis the difference is irrelevant as the difference could be explained by ...as TIGHTCHOKE  suggested a slightly sharper incline, slghtly harder accelration at some point would make the difference.  I know many many years ago there was  a deal where water was injected in small quantities because it was believed it increased the bhp.    Tyres are also going to drag less on a dry relatively warm road surface to a cold wet surface.  The percentage difference is very small anyway. and only equates to a litre or so of fuel over a short distance.

     

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    I did consider a number of factors including running on supermarket petrol on the way back .To be frank when you do a 220+ mile drive averaging 59 mph { it takes me 35 minutes to do the 18 miles to the M5 } and getting 52/55 mpg I am not complaining , its just I have noticed differences in MPG over similar runs in the past so just a passing thought .

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    In the days when I had a 1931 Austin 7 to drive, the difference in "performance" between cold wet days and hot dry ones was very marked - it always ran better in the wet.

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    A diesel will perform better in cold weather because of the colder air going into the inter cooler .it was always said that a petrol engine will perform better in wet weather ,

    harnser

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    My Vw T6 will get 51mpg at 50 mph on a run but 31 locally.

    in the last cold spell on a trip to work where I get 27-30 mpg I was getting 16-19 mpg :/ . 

    This van is now a year old and after speaking to the Vw assist man he said that’s diesel for you.

     

    what I do know is when it’s cold my van is crisp and quicker.

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    Cold air is more dense so you get more squeezed into the cylinders hence more bang, better power and economy. If you travel up and down the country during the night no the air is cooler with higher humidity, you notice your mpg and how much better the car drives.

    hot summer days are pants for mpg and power. 

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    3 hours ago, figgy said:

    Cold air is more dense so you get more squeezed into the cylinders hence more bang, better power and economy. If you travel up and down the country during the night no the air is cooler with higher humidity, you notice your mpg and how much better the car drives.

    hot summer days are pants for mpg and power. 

    Diesels are terrible In the cold for mpg but I agree run better 

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    I don’t mean cold like in winter. But in warmer weather during the night when it’s only 4 or 5 degrees. 

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    ALL meteorological conditions will affect the power output and therefore the fuel usage on any combustion engine that has to suck in atmosphere.  (All of them).

    Every pilot and aviation engineer and designer is well aware of the problems incurred by changes in the airmass.   Water methanol injection has been used as a power boost since WW2 for high performance aero engines.   Now much less common.

    Headwind/tailwind will affect things too as will road surface, tyre condition and pressure and the cleanliness of the outside of the car.   Weight, occupants and/or cargo, and fuel load all have there place in the calculations too.  

    No I am not going into the technicalities - I am retired.   Email Adrian Newey at Red Bull Racing.

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    3 hours ago, Grandalf said:

    ALL meteorological conditions will affect the power output and therefore the fuel usage on any combustion engine that has to suck in atmosphere.  (All of them).

    Every pilot and aviation engineer and designer is well aware of the problems incurred by changes in the airmass.   Water methanol injection has been used as a power boost since WW2 for high performance aero engines.   Now much less common.

    Headwind/tailwind will affect things too as will road surface, tyre condition and pressure and the cleanliness of the outside of the car.   Weight, occupants and/or cargo, and fuel load all have there place in the calculations too.  

    No I am not going into the technicalities - I am retired.   Email Adrian Newey at Red Bull Racing.

    This

    We had to optimise the Olympus TM3B gas turbines on the good ship Invincible, wherever we went in the world.

    It was a ballache, but we couldnt have the RAF Harriers dropping into the sea at the end of the ski ramp just because we couldnt do 30 knots into the wind:lol:

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    Also remember that modern diesels with a DPF continuously monitor and periodically increase the amount of fuel going in to make the exhaust gases burn hotter. This in turn burns off the soot build up in the DPF.

    When this is going on your MPG will be rubbish.

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    On 12/04/2018 at 18:43, Gunman said:

    I did consider a number of factors including running on supermarket petrol on the way back .

    Supermarket petrol is possibly the explanation. I know someone sad enough that he compared branded fuel with the supermarket variety. He claims that branded fuel cost him less because he got noticeably better mpg. His theory is that Esso, Shell etc. put in additives to boost the octane whereas the supermarkets just sell plain old petrol straight from the refinery.

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    On 12/04/2018 at 18:07, Medic1281 said:

    Might just be the fact you had ahead wind going one way!

    Exactly that. I travel from near Glastonbury to Lechlade and back regularly. The prevailing wind is behind me on the way up and against me on the way back. I get lower MPG on the way back. Think of it, you generally get better MPG at 60mph than you do at 70mph, so 60mph with a 10mph tailwind is the same resistance as driving at 50mph. Driving at 60mph with a 10mph headwind is the same resistance as driving at 70mph.

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    Today all petrol sold has 20% ethanol in it and diesel has 20% bio fuel mixed in. The government past legislation allowing this a few years ago. This means that neither fuel is pure and therefore you won't get perfect analysing results. Also means your dpfs, lambda sensor and fuel filters go wrong more and cause emissions issues at tests. 

    Helps to get older cars off the roads and new ones out of the showrooms though.

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