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Discussion Starter #1
Hi, I was averaging 41.5mpg prior to changing my tyres from the standard Goodyear runflats.

After I fitted 2 x Pirelli P Zero 245 40 R 18 93Y run flats I am now getting 28mpg on average.

Questions:
- How do I know what pressure these should be inflated to?
- Should I use the OEM pressures as per the Goodyears previously fitted.

I have read the threads about the thermostat issues and appreciate that this could also be the problem but before I go down this route I just wanted to understand if the tyres / pressure could be causing this.

Any advice appreciated.

Thanks
Phil
 

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hello mate that doesnt sound like a tyre thing the p zero's are a better quality than the goodyears its a huge difference in mpg, the tyre pressure i use is the recommended one by bmw (just by the door) im experiencing the same problem but it allways occurs on cold weathers im currently on 28mpg (non motorway) and thats a 530d
 

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its a funny thing i thought the colder the air the better the burn and so better mpg but ive had loads of cars around 30 in 12 year ( i know its a good job i dont go through women at the same rate ) but the last couple were the mondeo st tdci and it was the same every winter and i do alot of town driving but on av the mpg droped down to 29 mpg
 

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its a funny thing i thought the colder the air the better the burn and so better mpg but ive had loads of cars around 30 in 12 year ( i know its a good job i dont go through women at the same rate ) but the last couple were the mondeo st tdci and it was the same every winter and i do alot of town driving but on av the mpg droped down to 29 mpg, my 525d is at 27 atm
 

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Doubt it is the tyres, factor in that oil gets much thicker when cold and your axle, engine and gearbox all contain oil, not to mention grease in wheel bearings and drive shafts. Then add the extra energy used to power lights, heated windows, seats, heater blower and finally the engine needing to run richer and the cold weather is a recipe for disaster when it comes to fuel economy! Tyres only change a tiny amount in rolling resistance between brands.

Just the timing is bad I reckon ;)
 

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I would agree tbh,our bimmer dropped to around 30mpg over the cold weather and is now back up to 35mpg.Thank God at these current fuel prices.
 

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I agree with you guys.
Different tyre brand do show relatively small differences in rolling resistance but what you have seen is huge......

You should be looking for some other problem like engine temperature as you already identified

Good luck
 

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Try using main line fuels, Esso, Shell etc rather than Supermarket fuels. The Petroleum Companies use a winter grade diesel and the Supermarkets are notorious for either not bothering or supplying a patchy service.
 

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Discussion Starter #9
Ok so checked tyres pressures and (fronts were a 1-2 PSI low and rears were 6 psi low).
All of a sudden MPG is back up again 38mpg after a short run yesterday.

But ....its also a lot warmer so the temp could also have come into play.

Hopefully this means its not the stat and is just me being stupid.

Thanks for all of the advice.

Phil
 
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