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Discussion Starter #1
GOAL:
For my application (cheap track car for 24 Hours of Lemons), I was interested in increasing acceleration at the expense of top-end. After all, given how curvy Infineon is and the fact that we have a 3.8L n/a, who cares?


Obviously, changing the rear gears would help with this issue but it would also introduce another point of failure (i.e.: me messing something up that currently works without any fuss)

While I knew that my moving to smaller diameter tires would change my effective final gear ratio, I didn't have a way to quantify this change.

This factor must be weighed against
a) the overall stickiness of the new tires compared to stock
b) their ability to last the race given the vehicle's weight
c) the fact that a smaller diameter tire will adversely affect the size of my contact patch (due to a lowered roll center).

Calculates Tire spec differences
http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireMath.jsp

Calculates difference in final drive ratio
http://www.4lo.com/calc/geartire.htm

Note: The industry rule of thumb is that for every 1/2" change in rim width, the tire's section width will correspondingly change by approximately 2/10"


What I simply did was aggregate the following items:
* treadwear rating
* ancedotal feedback (such and such works well on tbirds/heavy cars in lemons, these tires don't)
* measured section width from the tire spec (use the rule of thumb to convert and get an indication of the contact patch size)
* New effective final drive ratio (to gauge acceleration)
* New top governed speed (assuming 108MPH for the stock size); can be defeated with a new speedo gear.
* ultimately cost.

We narrowed the field down to 2 choices.
- Toyo Proxes 4 (PX4) in 225/50-R15
- Dunlop Direzza DZ101 in 205/55-R15

Both have a higher treadwear rating than the Star Specs but this isn't necessarily a bad thing on a heavier car (and still better than the stockers), they are taller so the drop will be more minimal, the section width will offer a contact patch the same size or larger than the original tire size, and ultimately both are still smaller than stock so we will see an effective final drive ratio of 3.68:1 vs. 3.27:1

Now, I'm letting the team decide if the PX4's are worth a ~$70 premium per set.
Just thought I should share...
-g
 
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