Effect of Tire Diameters

Decoding 6UL 15”x9” 225/45R15 – Part 1 of 3: Effect of Tire Diameters

With the installation of the 6UL 15”x9” wheels from 949racing to replace the Rota Grid 16” x 7” rims,  in our opinion, the appearance of the MSM has transformed from hot to scorchingly sexy.

We are very glad that the obnoxiously embarrassing red lips on the Rota which appeared like someone purposely scraped the rims on some red curbs to engrave that ricey lip are gone.  However, we will stop here since appearance is a subjective thing.

Ride and handling characteristics, and overall driving experience on the 6UL have noticeably changed.  Is that a change to the improvement side of things or deterioration? How does the 6UL affect overall gear ratio, moment of inertia, and unsprung weight? Let’s dissect the components objectively to find out.  In this part 1 of the 3 parts series, we will look into how tire diameters affect overall gear ratio.

Background Info

Contact patches are now served by a set of 225/45ZR15 (22.1 lbs) Toyo Proxes R1R Extreme Performance Summer tires versus 205/50R16 Dunlop Direzza Sport Z1 Star Spec Ultra High-Performance Summer tires on the Rota rims.

For those who can’t recognize the meaning of 6UL 15”x9”225/45R15 by now, please return to your favorite cartoon network.

Dissect #1 – Effect to the Changes in Tire Diameter

A. Basis of Analysis

From the table on the left, the tires on the Rota have 2.5% and 4.6% larger diameter than stock and 6UL, respectively.  The tires on 6UL is 2.1% smaller than stock. Also noted in the columns for revs/mile, a smaller tire rotates more (revolutions) to travel the same given distance due to decrease in circumference (i.e. the 6UL has to turn 878 times to travel a mile vs. 838 in the Rota).

Sidewall thickness between Rota and 6UL has magically matched. Therefore, it has eliminated another variable in analyzing ride comfort and handling characteristics. The only other variable is sidewall stiffness between these two tires but that is a separate topic.

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