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I was bored today at school (what else is new) so i figured out what our effective gear ratio would be for the popular tires we run. Our stock tires are 24.49 in diameter and the stock final drive is 4.41:1, so if you switch to the autox favorite 205/50/15 it is like changing to a 4.68:1 gear. but if you are like most of us and bought 17's first and wonder why the 215/45/17 seem slugish off the line, it's like having 4.38:1.
happy motoring.

 
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i guess it might not be that interesting

 
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Its not that it isnt interesting... theres just not a whole lot we can do about it....

 
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just thought i'd share.

 
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I never understood why people go with 215 45 17's alot. I went with 215 40 17's because I like the sidwall and there a little smaller than a stock EX wheel. But you still feel the weight a bit.

 
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Good write up.

I had 215/45/17's on my Civic and they always felt slow to me as well.

DaddyFatSacks...people go with them in the first place to get more rubber between the wheel and ground. Makes for a better rie and in my opinion looks better.

 
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now im not an auto x guru, nor do i know how to convert tires size into final drive ratios, but wouldn't a 16" wheel give to proper balance of sidewall and rotational mass? unerstadably a 15 is going to be quicker to get going, but the advantage of a lower sidewall in cornering make for a better auto x car... ?

 
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The issue with side wall flex really only applys to street tires.... the tires we use for autocross be them "Race" compounds or the "Street" tires used in STS have Much stiffer side walls.

Example, one of the main "Street" tires used in STS classed autocross is the Falken Azenis sport. Falken, in an effort to reduce side wall flex, added steel belts in the side walls.... My 17's that I use on the street 215/45-17 are subsantually less responsive (i.e., have more side wall flex) then the Falken Azenis I use for autocross in 205/50-15...

if you go to "R" Compound tires such as the Kumho Victoracer or a Hoosier tire, then side wall flex becomes even less of an issue.

 
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^^^^^^^^^^^

well... given that information... what is the disadvantage of using a 16" wheel/tire? (im always in the mood to learn)

 
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you'll lose a bit of acceleration, thanks to a slight increase in rotational inertia.... thats pretty much it

 
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if you provide a tire size i can tell you what the gear change would be

 
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Zzyzx, your knowledge amazes me

 
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more calculations from boredom
Stock civic Drive Wheel Torque (based on stock 115 pounds of torque & stock gears)
1355.491 lbs ft
Stock Wheel Thrust- 1328.91 lbs
With autox 205/50/15- 1406.11
With 215/45/17- 1320.28
Stock G's of acceleration- .518 (no wonder why a civic can't throw you back in the seat)
W/ Autox tires- .548
W/ 17's- .5145
this would explain why the 17's seem kinda slower but everyone says the 15 accel much faster.

next installment in a couple of weeks when i can scale my car and figure where the center of gravity is and how much weight we transfer.

 
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Are you taking a class on Automotive mathmatics or something???

 
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i go to school for high performance technologies. and right now im in high performance drive lines, so there you have it.