[JRMA85BMW]

I have just replaced all my run flats for Kuhmo PS91 tyres 18". Just wanted to do this to get a smoother ride.

One thing I am unsure about is should I use the same 32PSI all round the run flats were? Or should I increase the PSI?

I did notice my fuel economy suffered when I had the Kumho were 32PSI so I upped them too 36PSI all round now.. The fuel is good now but the ride is very hard!

Maybe I need to play and try going down to 34/35 psi and see if this makes a difference..

Just wondering what people views were on this and what people have set theirs too when replacing the run flats for normal tyres?

[Hooded]

I tend to add about 2psi extra with non run flats due to the side walls not being so hard. So if you were running 32psi before I would try 34.

[Eddamoo]

Originally Posted by Hooded

I tend to add about 2psi extra with non run flats due to the side walls not being so hard. So if you were running 32psi before I would try 34.

If anything, you should REMOVE pressure due to the sidewall being softer.

The reason being that with a stiff sidewall, you have to inflate the centre patch of the tyre extra so the edges dont bear the weight. This is why RFT's are so prone to shoulder wear when under inflated.

The best example of this is on the early Z4's, where the //M model had ~4PSI less in its regular tyres compared to the 3.0i regular model with RFTs. The M was also heavier due to an Iron block S54.

I would go 2-4PSI under the stated pressure and have a play with it, or seek out the M3's details (provided it uses normal rubber!).

[JRMA85BMW]

Originally Posted by Hooded

I tend to add about 2psi extra with non run flats due to the side walls not being so hard. So if you were running 32psi before I would try 34.

Thanks. I know the Kumho though have hard sidewalls. But maybe I will drop to 35 psi and see first.. Just I am feeling 36psi is too rough. And I know with m sport suspension is very hard anyway.

[JRMA85BMW]

Originally Posted by Eddamoo

If anything, you should REMOVE pressure due to the sidewall being softer.

The reason being that with a stiff sidewall, you have to inflate the centre patch of the tyre extra so the edges dont bear the weight. This is why RFT's are so prone to shoulder wear when under inflated.

The best example of this is on the early Z4's, where the //M model had ~4PSI less in its regular tyres compared to the 3.0i regular model with RFTs. The M was also heavier due to an Iron block S54.

I would go 2-4PSI under the stated pressure and have a play with it, or seek out the M3's details (provided it uses normal rubber!).

Originally Posted by Eddamoo

I would go 2-4PSI under the stated pressure and have a play with it, or seek out the M3's details (provided it uses normal rubber!).

When I was running them at 32psi the car felt 'heavy' and my mpg took a hit. I even was driving the car same style I used to with run flats. Soon as I inflated to 36psi the mpg has returned good but the ride feels now too hard.

I guess it's trying to find that in between....

I was reading the Kumho PS91 have a hard sidewalls and are also a 'sticky' tyre. They rate F on fuel where my Bridgestone tyres were like a B I think for the run flats.

[HighlandPete]

Originally Posted by JRMA85BMW

Thanks. I know the Kumho though have hard sidewalls. But maybe I will drop to 35 psi and see first.. Just I am feeling 36psi is too rough. And I know with m sport suspension is very hard anyway.

Tyres should run at a similar pressure, if the L.I. rating is the same. Then adjust if necessary, for the best balance of handling/chassis dynamics vs. wear patterns.

[JRMA85BMW]

Originally Posted by HighlandPete

Tyres should run at a similar pressure, if the L.I. rating is the same. Then adjust if necessary, for the best balance of handling/chassis dynamics vs. wear patterns.

Thanks everyone. Will give it a try and see what it's like lowering the pressure. I also got the engine remap done last week so lots more torque.

[Jeff123]

As the plate on the door jamb. Don’t overinflate.

[Hooded]

Originally Posted by JRMA85BMW

Thanks. I know the Kumho though have hard sidewalls. But maybe I will drop to 35 psi and see first.. Just I am feeling 36psi is too rough. And I know with m sport suspension is very hard anyway.

I personally run 35psi front and 37psi rear on the M Sport 18" wheels with non run flat tyres.

[JRMA85BMW]

Originally Posted by Hooded

I personally run 35psi front and 37psi rear on the M Sport 18" wheels with non run flat tyres.

Is there a reason why you go higher on the back then then front? The sticker on my door says same psi all round for the run flats..

[HighlandPete]

Originally Posted by JRMA85BMW

Thanks everyone. Will give it a try and see what it's like lowering the pressure. I also got the engine remap done last week so lots more torque.

Just a thought, what spec' and ratings have you fitted, standard to similar ratings, or XL with a higher load rating?

[JRMA85BMW]

Originally Posted by HighlandPete

Just a thought, what spec' and ratings have you fitted, standard to similar ratings, or XL with a higher load rating?

They are similar to the run flat ratings.

[HighlandPete]

Originally Posted by JRMA85BMW

They are similar to the run flat ratings.

Keeps it simple. Also remember torque distorts tyres, so don't run too soft on the rear, if you are using it often, or you'll wear them off faster.

[Hooded]

Originally Posted by JRMA85BMW

Is there a reason why you go higher on the back then then front? The sticker on my door says same psi all round for the run flats..

The sticker on my door (330d) shows the rears (255/40/18) should be a little higher than the fronts (225/45/18).

[JRMA85BMW]

Originally Posted by Hooded

The sticker on my door (330d) shows the rears (255/40/18) should be a little higher than the fronts (225/45/18).

I see. My sticker says all tyres should be same..

[Hooded]

Originally Posted by JRMA85BMW

I see. My sticker says all tyres should be same..

Just to add, I also think my car feels right with 35 front and 37 rear.

[HighlandPete]

Originally Posted by JRMA85BMW

I see. My sticker says all tyres should be same..

I did say remember the extra torque.

BMW will be allowing for that in the 330d setting pressures.

[JRMA85BMW]

Originally Posted by HighlandPete

I did say remember the extra torque.

BMW will be allowing for that in the 330d setting pressures.

I have just been outside and done my tyre pressures before using my car today. I put 35psi in fronts and 36psi in the backs to see how it goes.

The tyres I have are:

2 x Kumho Ecsta PS91 255/40 R18 Y (99), Reinforced
2 x Kumho Ecsta PS91 225/45 R18 Y (95), Reinforced

And the sticker from my car I have attached on this post to see what you think..

[JRMA85BMW]

Originally Posted by JRMA85BMW

I have just been outside and done my tyre pressures before using my car today. I put 35psi in fronts and 36psi in the backs to see how it goes.

The tyres I have are:

2 x Kumho Ecsta PS91 255/40 R18 Y (99), Reinforced
2 x Kumho Ecsta PS91 225/45 R18 Y (95), Reinforced

And the sticker from my car I have attached on this post to see what you think..

So been running 35 front and 36 backs this week but don't feel right. Front feels 'heavy' and mpg is better but not great. I am hitting 47.7mpg but on run flats I was getting 49mpg.

Decided to try next week 36 front and 37 back to see how that feels for the week. Will report back :-)

I don't want to go too high because ride will be very rough, plus I don't want the tyre to be wearing uneven if the whole tyre isn't making contact with road.

[Jeff123]

I am running those sizes at 34 front and 37 rear. Once they get warmed up you can add 2 PSI to that, meaning that the rears at just short of 40. Most of the time it’s just me in the car. That feels reasonable to me.

[JRMA85BMW]

Originally Posted by Jeff123

I am running those sizes at 34 front and 37 rear. Once they get warmed up you can add 2 PSI to that, meaning that the rears at just short of 40. Most of the time it’s just me in the car. That feels reasonable to me.

Originally Posted by Jeff123

I am running those sizes at 34 front and 37 rear. Once they get warmed up you can add 2 PSI to that, meaning that the rears at just short of 40. Most of the time it’s just me in the car. That feels reasonable to me.

Thanks Jeff! will see what next week on my drives feel like