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<02> Tire Size Chart Errors

Doug Falls <falls@domain.elided> pointed out a grievous
and serious error in my 2002 tire size chart: 

> the 165/R-13 is slightly taller than the 185/70-13 at 23.4
> vs. 23.2 [inches] respectively.

Since I used the wrong stock diameter and since the order of 
sizes and percent difference from stock in the table are based 
on the stock diameter, the stock size was out of order, and 
_all_ the 'percent' numbers were wrong.  

Doug continues:

> From my calculations, the 185/65-14 is the best 14-in choice, 
> as it is the nearest to stock at 23.47.  Second best 14-in is 
> the 195/60-14 at 23.2 ... I am considering using E30 (14x6) 
> alloys with 185/65HR-14's on my '02 because I really want my 
> speedo to be accurate.  I am staying away from 195's or 205's 
> because I have heard steering is hard at slow speeds.

Bill Ozinga of the Tire Rack confirms that 23.4 inches is the
correct diameter of the stock size:

> Your mistake is the 0.70 should have been 0.80.  When the 
> aspect ratio isn't listed, it's assumed to be between 78-82; 
> for mathematics sake, it's usually estimated at 0.80.

And suggests this size for 14-inch wheels: > Use the 195/60-14's.

Well deserved plug for the Tire Rack:
 ------------------------------------------------------------------
 Bill Ozinga - oz@domain.elided
 Webmaster - The Tire Rack 
 http://www.tirerack.com
 Ph. (888)362-8473 or (219)287-2345 ext. 367
 ------------------------------------------------------------------

The corrected and expanded chart follows:

                                      (2)
                                      Diff.
                               (1)    from
 13-inch   14-inch   15-inch Diameter stock     
  Wheel     Wheel     Wheel  (inches) (%)  Comments
*-------- --------- ---------  ----  ----  -------------------------------
          195/55-14            22.4  -4.1  (3,4,5)
175/70-13                      22.6  -3.5
205/60-13                      22.7  -3.0  (7,11)
          185/60-14            22.7  -3.0
                    195/50-15  22.7  -3.0  
          205/55-14            22.9  -2.2  (7)
          175/65-14            23.0  -1.9
195/65-13                      22.9  -1.8     
                    185/55-15  23.0  -1.7
          165/70-14            23.1  -1.3   
                    205/50-15  23.1  -1.3  (7)
          195/60-14            23.2  -0.9  Stock for some E30 (3,8,11)
185/70-13                      23.2  -0.9  Stock for 2002 turbo

165R-13                        23.4   0.0  Stock '02 size (except 
                                           2002 turbo) (11)
                    195/55-15  23.4   0.0
205/65-13                      23.5  +0.4  (7)       
          185/65-14            23.5  +0.4
          205/60-14            23.7  +1.2  (7)
	            185/60-15  23.7  +1.5 
                    205/55-15  23.9  +2.0  (7)
175/80-13                      24.0  +2.5  Rubbing likely
          195/65-14            24.0  +2.5  Stock for most E30 
                                           (3,4,5,6,8)

Notes:

(0) Some sizes may not be available.  Check tire weight rating 
before use.
(1) Diameter is approximate.  It varies with tire manufacturer 
and model.  It also decreases about 1.1 % from new tread (10/32 
inch) to no tread (2/32 inch).
(2) Assumes stock tire size of 165R13, aspect ratio of 0.80, 
and diameter of 23.4 inch.  Differential ratio is effectively 
reduced by this amount, e.g. with 195/65-14 tires the stock 
3.64:1 ratio becomes 3.55:1.  Speedometer reading is reduced by 
this amount, e.g. with 195/65-14 tires the speedo reads 63.4 at 
65 MPH.  Speedo reading increases as tires wear (1).  Speedo 
reading varies with normal manufacturing tolerances, typically 
- -2/+5 % IMO.
(3) Slight, occasional rubbing on front inner fender walls at 
full lock is common with these sizes.
(4) Slight, occasional rubbing on tie rod boots possible on tii.
(5) Slight, occasional rubbing on front fender lips is common.
(6) Slight, occasional rubbing on front apron is possible.
(7) Rolling, trimming, or stretching of fender lips is required; 
rubbing is likely.
(8) E30 ('84-'91 3-series) 14-inch alloy wheels fit '02s well and 
are a common upgrade.  If tires are included, try for 195/60 
rather than 195/65.  Replace steel wheel nuts with alloy wheel nuts.
(9) E21 320i wheels fit '02s poorly because of their different 
offset.
(10) Width:  At the stock diameter (23.4 inch), a wider tire will 
have less clearance; use a smaller diameter for more clearance.
Steering effort and pavement groove wander increase with width.
(11) Most popular sizes.

Comments and corrections are appreciated.

Curt Ingraham
ingraham@domain.elided