Hi guys, very interesting observation Karan & iRaghava...
These are not comprehensive or rigorous, but, by switching to a taller tire (~7.3 mm), the torque arm has increased in length, but not by much. The gearing at the contact patch decreases from 3.189 to 3.152, so the drop is very very small (~1.2%).
Coming to aero drag, the section width has increased by 20 mm due to the tire change. Assuming a ground clearance of 16 mm, & even allowing for a Cd of 1.0 (way too high as the tire curves away), the aerodynamic force due to the increased width alone, at a speed of 200 Kmph, is around 1.1 N only.
If you ask me, I don't think increased aero drag due to the wider tires alone can explain the relatively large drop (> 5%) seen in the top speed. The tune of the car could've decreased or other mods (like a body-kit) could be at fault. The rolling resistance could've increased too (although I think stiffer tires have lower rolling resistance values). Other things to consider - the tire pressures could've been lower, or the tread could've been thicker !
Last edited by im_srini : 14th October 2008 at 11:58.
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