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Thread: Do petrol mpgs variy as much as diesel?

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    Regular Member bigdave1701's Avatar
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    Default Do petrol mpgs variy as much as diesel?

    On a short cold engine run a 1.9cdti diesel engine will do 35mpg but on a long run the same engine will get 55-60mpg. That's as near as dammit twice the economy and I know petrol's aren't quite as economical as diesel's but do they have such a huge difference between runs? Will a 2.0T do 25mpg on a cold engine and then 45-50mpg on a run or is it all much of a muchness?
    Last edited by bigdave1701; 21st March 2012 at 12:20.

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    Regular Member bigdave1701's Avatar
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    Sorry, just realised the typo in the title! should have been vary (dont know where the i came from)

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    Regular Member Neve'o's Avatar
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    Vehicle : Vectra SRi 150 Hatch

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    Engine : Z19DTH

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    Im sure petrol's will not vary so much, my old z20let running 260bhp would do around 31/32mpg around town & driving to work, on a run to say a car show it would do around 36/37mpg. My current CDTi 150 is doing 42mpg around town / work and hope to be much much more on a long run...

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    Regular Member rich r's Avatar
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    I would say that depending on driving style and conditions you can get huge variations in fuel economy on the petrol models.

    If I drive at a steady 56mph on a smooth flat road, I can get over 50mpg (55mpg according to the BC, but I reckon nearer to 51mpg based on how much fuel I put in). However if I have a car full of stuff and drive up and down steep hills, accelerating and braking a lot (eg going across the North York Moors), I've seen it as low as 25mpg average over a couple of hours.

    On my old Subaru Legacy GTB (twin turbo 300bhp), I could get 35mpg on the motorway driving sensibly, or under 10mpg on track days or when drag racing. When I had a Nissan Pulsar GTI-R, I could physically see the petrol gauge moving downwards when blasting round back roads. Lots of hard acceleration and braking is one of the biggest killers for economy.

    However - my Vectra is a 1.8 Dual Fuel Elegance, so I rarely run on petrol. Performance on LPG is affected in the same way though. I can get 40mpg if I drive sensibly, or drop it to about 20mpg if I have a full car and drive like a fool. But then - LPG's only 68p a litre, so it doesn't feel so bad

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    Regular Member m8internet's Avatar
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    You've pretty much answered your own question

    A short run with the engine still cold with be a lower MPG

    A longer run with the engine warm will be a higher MPG

    The short run is typically at lower speed, the longer one at higher speed
    If you want to compare like for like, then perform them at the same speed and compare (such as 40mph)

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    its the stop start and cold engine driving that kills the diesel mpg- once they are warm or on a run the diesel comes into its own.

    I had a 1.6 petrol focus that always got about 35mpg around town and only ever got 37mpg on long runs. around town a petrol does low revs but to get motorway speeds the engine has to rev much higher and the faster the engine runs the less efficient it is.
    maybe this explains why city cabbies like the 2.2 direct as around town it gets about the same as diesel for lower fuel costs but more rural cabbies prefer a diesel......

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