<VV> 175 vs 185

[BobHelt at aol.com]

In a message dated 9/1/2006 9:29:33 AM US Mountain Standard Time,  
bowtieguy at cox.net writes:

I  thought the 175 was closest in size diameter to the 650:13 from past  
discussions for speedometer accuracy. What did I miss?

Bob  Vukas



Hi Bob,
As with all things in life, one must define his terms. Otherwise we start  
comparing apples and oranges.
 
First comparisons between stock (came on the car) tires and replacements  can 
generally be based on one of two characteristics (ignoring things like ride  
and cornering). 
These are 1) comparison of tire measurements of tires mounted on rims and  
filled with air.These wsould include diameter and circumference, etc.
 
Second characteristic 2) would be how many revs per mile the tires go when  
mounted on the car at say, 50 mph. This would affect speedo accuracy and other  
things.
 
These characteristics are entirely different because of the change from  bias 
tires on the 1960s and radials of today.
 
So here are the answers: 
 
                                                6.50-4ply        6.50-2ply    
   175          185
diameter  (Unmounted)            24.3                24.3              24.1   
     24.6
revs per mile  (RPM)                 853                 864               
863         843
 
So you can see that 175 tires are similar the the 2-ply in RPM but the 185s  
are closer to the 4-ply 6.50s.  So the choice now becomes less distinct.  
Similarly, the 175s are smaller in diameter than the 6.50s and the 185s are  
larger. So which do you choose? This affects how the tires fill the tire well  and 
how they "LOOK" when on the car. Most people choose the 185s for appearance  
and for being closest to the original 4-ply tire RPMs to keep the speedos  
reading close.
 
Regards,
Bob Helt.