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Discussion Starter · #1 ·
I know this was discussed once before, but I wanted it to be specific to the 981.
Do the larger wheels cause the engine to lose torque and thus actually reduce top speed ?
Or does it provide an increase ?
 

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No significant change, as long as the factory recommended tire sizes are used. The larger wheels come with tires that have a "shorter" sidewall, also known as aspect ratio. Bottom line, the rolling radius of all wheel sizes are very close.
 

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In my experience narrow tires will give you a higher straight line speed. Less air and rolling resistance. Hardly noticeable or practical.


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Discussion Starter · #4 ·
Less air and rolling resistance. Hardly noticeable or practical.
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Does that "hardly noticeable" also apply to lighter lug nuts ? i.e. not really noticeable at high speed ?
 

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Lighter lug nuts correlate to the car's unsprung weight. Lighter wheels, tires, brakes, etc. improves handling and acceleration. For the majority of us not really noticeable.....
 

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And Weight furthest from the axle will have a greater effect than weight closer to the axle. So a reduction of 1lb of tire will have a greater effect on braking and accelerating than 1lb of wheel bolts.


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Assuming you keep the rolling radius the same (you should), typically, a larger wheel (with a shorter aspect-ratio tire) increases unsprung weight, which is detrimental for all aspects of performance. But as mentioned, not in a meaningful way with so little difference in factory options. The ideal wheel size for our cars is 19" IMO, but guess Porsche wanted to go for the extra profit of bigger wheels, since they're in vogue;). Oh well.
 

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I would not go 180 MPH or higher (with gearing) in a Boxster, personally. It's not that I don't trust the car. It's that there aren't many places with 3+ mile straightaways where that could be remotely safe. If you could find some deserted autobahn at an off-hour in an off-season when it wasn't raining in Germany.
 

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You guys are all too technical. Smaller wheels open the window to quicker lap times because you can use Hoosiers! Finding a good track tire is nigh impossible if you have 20" wheels along with the staggered fitment.
 

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You guys are all too technical. Smaller wheels open the window to quicker lap times because you can use Hoosiers! Finding a good track tire is nigh impossible if you have 20" wheels along with the staggered fitment.
You are right, but since the fashion of 20"+ wheels is still here, all we can do is try to educate. Car & Driver did a test a few years ago using a Golf comparing wheel sizes from 15" steelies up to 19" alloys. Same tires, as much as possible. 17" gave the best lap time, and the 15 best acceleration. Steelies were the lightest setup,too. Who knew?


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My OEM is 235/265-40R19, I once run 235/265-40R18 and you can feel the torque (acceleration) difference. Can't tell you if top speed is difference because I can't find a place to test it out :). Anyway ... let's keep in mind these two basics: torque is how far you push a wall and hp is how fast you hit the wall so for how fast you go...

The specs for my boxster is: http://press.porsche.com/vehicles/2013/2013_Boxster_Specs_Final.pdf
315 hp at 6700 RPM
3.89 final drive ratio
.81 ratio in top gear
173 top speed

Plug the number into this web Potential Speed Calculator and the calculation said 172.76 ~ 173 so the Boxster S is limited by gearing, not air resistance or hp

So in theory if you have a slightly taller tires with the same width and same resistance then you will have have slightly higher top speed assuming air resistance not significant enough to sap all your available reserve hp.

If you put the PDK specs in you have a higher top speed at 7 gear but the specs said 172MPH still so what it means is the gearing of the 6speed is tuned perfectly for top speed and the PDK even with higher speed ratio can't overcome the deficit of hp and wind resistance.

Short answer is with smaller radius tire you have slower top speed and with taller radius tire you suppose to get higher top speed but you won't unless you invest in another 50hp or so.
 
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