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E36/7 Z3 (1996-2002)
E36/7 Z3 Roadster, Z3 coupe, Z3 M Roadster and Z3 M Coupe talk with our gurus here.

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  #1  
Old 11-22-2009, 06:58 PM
mhughett mhughett is offline
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Another (but different) tire question

My 3.0si sport package has the stock 18" run flat tires with 225/40's up front and 255/35's in back. When I bought the car, I knew they were run flats but thought they were the same size as what comes on the "M" Coupe. It wasn't until about a month ago that I found out that my car actually has a slightly lower profile (i.e shorter sidewall) than does the M. Is it possible that BMW did this since the sidewalls on the run-flats used on the 3.0si are stiffer that they provided more margin of safety to the rims vs. non run-flats?

Like many others, I'm going to be ditching the run-flats soon and I was thinking about getting the slightly larger M size tires for my car for a couple of reasons: 1) These would have slightly more cushion to protect the wheels, and 2) There seems to be several good tires I'm considering that are not made in the 3.0si size. I'm aware that with their slightly larger diameter that the spedometer will reflect about 3% lower speed than current tires but I'm not too concerned about this since it's common knowledge that BMW sets it spedometer to read about 3-5% too fast anyway. This would have the effect of "truing up the speedometer". Are their any other concerns I should have?

Thanks for your help.
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  #2  
Old 11-23-2009, 01:39 AM
Hoops432 Hoops432 is offline
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> I'm aware that with their slightly larger diameter that the spedometer will reflect about 3% lower speed than current tires but I'm not too concerned about this since it's common knowledge that BMW sets it spedometer to read about 3-5% too fast anyway. This would have the effect of "truing up the speedometer". Are their any other concerns I should have?

Common knowledge it may be but I didn't know that. I can't help with your question, but I found that when I went through a speed check recently with much bigger tires than standard, the speedo still overestimated my speed!! Speedo said 110kph but the "check-your-speedo" machine said 107kph. Maybe we need 19's to make the needle right.... ;-)

245/40ZR18 Dunlop Direzza Sport Z1 Star Spec Blackwall - Fronts
275/35ZR18 Dunlop Direzza Sport Z1 Star Spec Blackwall - Rears
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  #3  
Old 11-23-2009, 06:51 AM
mhughett mhughett is offline
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VMZ432:

Actually, your tires are SMALLER than the factory size for the Z4M when you are looking at the diameter, not the width. Since both are 18" wheels, the only variance is the width of the sidewall. If you take the tread width x the aspect ratio this will tell you the total width of the sidewall. Yours are actually slightly smaller than OEM size as shown below:

245 x .40 = 98 (yours)
225 x .45 = 101 OEM

275 x .35 = 96.25 (yours)
255 x .40 = 102 OEM
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2003 530i 5-speed, SP, PP, CWP, Xenon
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UUC short shifter with DSSR and ZHP shift knob
Michelin Pilot Sport A/S
BSW IQ speakers, Sirius
CDV delete
Huper Optik tint

2007 X3 3.0si
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  #4  
Old 11-24-2009, 05:08 AM
Hoops432 Hoops432 is offline
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Hey thanks! I probly should have known hey. Guess my needle is pretty good then. :-)

Anyone help the op?
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  #5  
Old 11-24-2009, 10:51 AM
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aestheticstorm aestheticstorm is offline
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Unless I'm missing something, the OP is asking a fairly obvious question. If the M's size tires are within 3% of your OEMs you should have nothing to worry about. A larger size can only help the "fabled" speedo error.
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  #6  
Old 11-24-2009, 01:32 PM
mhughett mhughett is offline
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I've seen this "as long as it's within 3%" rule thrown around a lot as it relates to tire sizes. Is there something sacred about 3%. I think the revolutions/mile for the M size tires are 3.5% less than the stock size. Does this mean there will be a problem?
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Bilstein Sports
UUC short shifter with DSSR and ZHP shift knob
Michelin Pilot Sport A/S
BSW IQ speakers, Sirius
CDV delete
Huper Optik tint

2007 X3 3.0si
6-speed manual, CWP, Xenon, Cft. Seats, Premium Pkg, Sirius, Privacy glass
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  #7  
Old 11-24-2009, 02:42 PM
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aestheticstorm aestheticstorm is offline
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I've heard it has to do with overloading your brake specs, but if the revs are actually slower than OEM (larger diameter), one would assume it may even improve braking slightly so long as your wheels/tires aren't significantly heavier.
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