New tyres....lower mpg

[mybeamerzinbitz]

Hi all...my first post here and I’m hoping you can give me some advice.
I have a 2015 320d xdrive which I’ve had for 8 months now.
Unbeknown to me at the time, I bought it with regular tyres and they needed changing this month so I changed all 4 to run flats which is what the car should have had I believe.
They are Bridgestone Potenza 225/50.
Have just done a 500 mile journey and the tank is nearly empty giving me only 51 mpg.
The same journey last month with the old tyres gave me 650 miles and around 60 mpg.
Has anyone any ideas what’s going on?

[cuprabob]

It could be down to the lower ambient temperature and nothing to do with the tyres.

[mybeamerzinbitz]

Thanks for your reply......So do you think the new tyres aren’t the cause of the higher fuel consumption?

[p1stonhead]

Tyres shouldn’t make much of a difference. Tyre pressures might. Are the high enough?

[mybeamerzinbitz]

Thanks for your reply......So do you think the new tyres aren’t the cause of the higher fuel consumption? 150 more miles to a tank of diesel on old tyres seems like a big difference which I’d why I’m concerned. Hubby says I’m worrying about nothing 🙄

[edthefed]

Last month the temperature was probably 10 degrees higher, the daylight longer, the rain and spray less etc all of which affect fuel consumption.
And check your tyre pressures...just because you have new tyres fitted doesnt mean the pressures are right

[VerySideways]

In my experience runflats actually give better fuel economy than normal tyres due to lower rolling resistance - but there are other factors at work here.
Tyre pressures, ambient temperature, etc. as others have already said... but also winter diesel. Diesel in winter is formulated differently than diesel in summer, as summer diesel when exposed to freezing temperatures turns to a sludgy mess, so winter diesel has additives to lower the freezing temperature. This also makes fuel consumption worse.

[Welshbeef]

Check tyre pressure.

You might have had low rolling resistance eco tyres as the old set and gripper more sporty new tyres

Winter diesel makes a difference likewise engine running whilst you defrost the car too makes some difference.

Rain / puddles cause drag


However the two big ones are tyre pressure - before could have been too high and these are correct pressure.

[Pica-Pica]

VerySideways said:

In my experience runflats actually give better fuel economy than normal tyres due to lower rolling resistance - but there are other factors at work here.
Tyre pressures, ambient temperature, etc. as others have already said... but also winter diesel. Diesel in winter is formulated differently than diesel in summer, as summer diesel when exposed to freezing temperatures turns to a sludgy mess, so winter diesel has additives to lower the freezing temperature. This also makes fuel consumption worse.

Echo all of that. New tyres would have a release agent to get the tyre out of the mould, that is why you are recommended to drive carefully. After that the tyres should be better for grip and economy.

Edited by Pica-Pica on Sunday 25th November 22:01

[Doofus]

mybeamerzinbitz said:

Hubby

fk off "rosanne"

[iain123]

My guesses!
The car thinks its travelled less distance as the tyres are probably 0.5cm bigger radius due to the additional tread depth. So the odometer indicates less distance which gives a higher consumption.
Like wise additional tread picks up more water so makes the tyres heavier and harder to turn.
Darker, colder nights means heated seats, head lights, heated rear windows, etc – all take energy from somewhere.

[Monty Python]

iain123 said:

My guesses!
The car thinks its travelled less distance as the tyres are probably 0.5cm bigger radius due to the additional tread depth. So the odometer indicates less distance which gives a higher consumption.
Like wise additional tread picks up more water so makes the tyres heavier and harder to turn.
Darker, colder nights means heated seats, head lights, heated rear windows, etc – all take energy from somewhere.

1cm extra diameter adds 3cm to the rolling radius - which is a about one tyre revolution less per kilometre. I doubt that'll account for a 9mpg decrease in mpg.

[iain123]

Oh, did I say it did? 3 cm is about 1% difference or 0.5mpg.

[Welshbeef]

Another thing - when was it last serviced? A freshly serviced car produces more power and is more efficient a car just due a service will be the opposite.

So tyre pressure
Eco tyre less rolling resistance v sportier
Winter diesel
Actually colder outside temp so running longer below normal operating temp as such running rich
Wetter meaning more puddles and wet road more rolling resistance
Traffic heavier due to winter conditions again impact
Is the tyre the same dimensions ? It’s a bigger circumference due to being new v worn out v
Is the tyre wider - more rolling resistance

Have you got a full boot
Have you been driving round with a fuller fuel tank
Have you been eating a lot of pies?
Have you been carrying other passengers
Have you left the roof bars on
Have you left the roof box on
Did you leave climate control on with air con all the time or only just changed

[GregK2]

Reject the new tyres. Demand the old tyres are refitted.

[Welshbeef]

What was the economy grade of the new and old tyres?

[mybeamerzinbitz]

Thank you alll for your advice and comments....it’s greatly appreciated.
I think I will take it to bmw dealer (take out a small mortgage first!) and get it checked out...it is due a service soon, might be as well to bring it forward, presumably they will check tyre pressure is correct then. Cheers everyone

[SteBrown91]

mybeamerzinbitz said:

Thank you alll for your advice and comments....it’s greatly appreciated.
I think I will take it to bmw dealer (take out a small mortgage first!) and get it checked out...it is due a service soon, might be as well to bring it forward, presumably they will check tyre pressure is correct then. Cheers everyone

your car has tyre pressure sensors in the wheels, you can check pressures in the idrive.