Z4 3.0 18" tire sizes

brandobot · 08-13-2010, 08:49 AM

Has anyone ran 225/45/18 in front with 255/40 in rears with any problem? Does the extra sidewall side make a huge diff?

vachss · 08-13-2010, 09:37 AM

Ask someone who's put Z4M wheels/tires on their non-M. Sidewall compliance will be greater and probably feel much like the 17's on a non-sport-package Z4.

But the biggest effect will be ~4% taller effective gearing. Your speedo will finally read just around accurately, but your acceleration in any given gear will be a bit reduced. Once you get up past 1st gear though this effect is largely negated though since you get to stay in lower gears to a commensurately higher speed.

brandobot · 08-13-2010, 01:21 PM

Interesting, so by actually putting the 45 and 40 series tires on my 3.0, it'll correct my speedometer?

vachss · 08-13-2010, 02:23 PM

Quote:

Originally Posted by brandobot

Interesting, so by actually putting the 45 and 40 series tires on my 3.0, it'll correct my speedometer?

When I measured it against a GPS I found that my 3.0si speedo consistently measured 4% high. So going from 25 to 26" outer tire diameter should just about compensate.

brandobot · 08-17-2010, 01:03 AM

Good to know, thanks!

Any reason why BMW would put a lower series tire when putting a 45 series would make the speedometer more accurate?

08-13-2010, 08:49 AM	#1
brandobot Second Lieutenant 41 Rep 291 Posts Drives: 06 Z4M Join Date: Jul 2010 Location: Bay Area Ca iTrader: (3)	Z4 3.0 18" tire sizes Has anyone ran 225/45/18 in front with 255/40 in rears with any problem? Does the extra sidewall side make a huge diff?
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08-13-2010, 09:37 AM	#2
vachss Captain 55 Rep 815 Posts Drives: Z4 Coupe Join Date: Jan 2008 Location: Ventura County, CA iTrader: (1)	Ask someone who's put Z4M wheels/tires on their non-M. Sidewall compliance will be greater and probably feel much like the 17's on a non-sport-package Z4. But the biggest effect will be ~4% taller effective gearing. Your speedo will finally read just around accurately, but your acceleration in any given gear will be a bit reduced. Once you get up past 1st gear though this effect is largely negated though since you get to stay in lower gears to a commensurately higher speed.
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08-13-2010, 01:21 PM	#3
brandobot Second Lieutenant 41 Rep 291 Posts Drives: 06 Z4M Join Date: Jul 2010 Location: Bay Area Ca iTrader: (3)	Interesting, so by actually putting the 45 and 40 series tires on my 3.0, it'll correct my speedometer?
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08-17-2010, 01:03 AM	#5
brandobot Second Lieutenant 41 Rep 291 Posts Drives: 06 Z4M Join Date: Jul 2010 Location: Bay Area Ca iTrader: (3)	Good to know, thanks! Any reason why BMW would put a lower series tire when putting a 45 series would make the speedometer more accurate?
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