Different tyre size combination, does it matter ?

[Risk.Take]

I was using 205/50 R17 RFT (both rear and front), on my 130i Exclusive. 2 days back I changed to 225/45 R17 (rear), and still have 205/50 R17 (front).

Now I keep on getting RunFlat alert, I reseted it after changing the tyres 2 days back. Yesterday, the RunFlat alert appeared again, then I passed by service station to check pressure, and there was no tyre with low pressure. Then I reset the RunFlat again. This morning the damn RunFlat alert appeared again.

My question is , is it because I changed to different tyre sizes now 205/50 R17 (front), 225/45 R17 (rear) ??? Or what can be the cause ?

Was is right for me to use the above-mentioned tyre sizes combination?

Cuz, this alert is really irritating the hell out of me now, as I have no tyre that has low pressure.

Please advise guys!

 

[Skwinty]

The flat wheel alarm is calculated on wheel RPM.

So, different size tyres will cause the problem as the wheels no longer have the same diameter. :sorry:

 

[Risk.Take]

Damn....! So, can it be fixed, if I can go straight to BMW Dealer, as in can they reset the thing to calculate on current diameter?

 

[moranor@axis]

there is only about a 2mm difference in diameter... you could try running a higher pressure on the back and see what happens...

im not sure the tire size is the problem here because 2mm is nothing... you could get more with tire wear...

 

[Risk.Take]

Current I was running air pressure of 230 (front 205/50), and 250 (rear 225/45) ........ can you recommend how much air pressure to put, to try to balance the diameters ?

 

[Skwinty]

It seems that small differences in diameter take quite a long time to appear as an alarm.

With my MINI, a slight drop in pressure, about .2 bar, causes the alarm to come up quite quickly, within 500 metres.

Must have something to do with sample rate and obtaining consistent readings.

 

[moranor@axis]

i got no idea, but check the sidewall marking to see the maximum pressure they can take so you dont go over that...

i would increase the pressure by 10kph till the warning goes away...

you dont want to run too much pressure because it will give you less grip and will also wear the middle of the tire faster...

i would also call up bmw and ask them if there is a better fix...

i really dont think 2mm should make a difference thou thats why playing around with the pressure would allow you to see if it could be the problem...

 

[Risk.Take]

Thanks, I will just pop by BMW Sandton cuz its around the corner , and hear what their views are on the this matter.

I swear this alert is so irritating i cant really pass by service station frequently cuz of this.

Thanks a lot for your views guys!

 

[moranor@axis]

be sure to post what you find so it can help others :rollsmile:

 

[Risk.Take]

I'ma do right that, when I'm back and got help, cuz these BMW Dealers folks sometimes don't know their story *loool*

 

[moranor@axis]

thats why i thought you should try pressure 1st so you know whats going on... next thing they going to change sensors and the problem will still be there...

 

[Skwinty]

http://www.tiresizecalculator.info/


Original tire
Sidewall: 3.99 in
Radius: 12.49 in
Diameter: 24.97 in
Circumf: 78.45 in
Revs/mile: 807.61

Sidewall: 4.04 in
Radius: 12.54 in
Diameter: 25.07 in
Circumf: 78.76 in
Revs/mile: 804.44

 

[moranor@axis]

i prefer this one:
http://www.1010tires.com/tiresizecalculator.asp

 

[Skwinty]

More data on that one.:thumbs:

 

[Raybimmer]

My Continental chart gives 633 mm for 205/50 and 638 mm for 225/45 so if the electronics count the wheel revolutions there will be a difference , the electronics pick it up as a pressure difference . IMHO the electronics cannot be programmed for different diameter tyres front and rear - how will it count tyre revolutions and know if a tyre is flat or larger/smaller diameter .IMHO deflating the larger diameter tyre will reduce its diameter , maybe enough to not alarm the tyre pressure monitor .