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Old 01-02-2007, 08:27 AM   #1
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Thinking of going to 18s. Will car essentially be slower from the bigger wheels?

I currently run with 17 inchers, but have been thinking about going to 18 inch wheels. I know the lateral handling will improve with this switch, but would this basically also slow my car down in a straight line because of the larger wheels, or is it negligable? TIA.
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Old 01-02-2007, 01:14 PM   #2
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When you upgrade to 18" wheels, you will also be running lower profile tires. While there may be a slight weight penalty for the larger rims and tires (ie heavier rims and tires) you may not really notice it in a seat of the pants scale. Also, what would have greater impact on acceleration is the overall diameter of the wheels.


For example - using my "fuzzy math" logic

A 255/40-17 would have effectively the same diameter as a 265/35-18

Tire = 255mm (width) x.40 (the aspect ratio) =102mm where 102 is the height of 1 sidewall

Rim = 17" = 17x 25.4 (where 25.4mm~ 1") = 431.8mm

So.. 2x102=204mm (2 sidewalls - one on the top and one on the bottom of the rim) 204mm + 431.8mm = 635.8mm


Tire = 265 (width) x.35 (aspect ratio) =92.75mm where 92.75mm is the height of one sidewall

Rim = 18" = 18x25.4mm = 457.2mm

So.. 92.75x2 = 185.5mm 185.5mm+ 457.2mm = 642.7mm

So, the difference between a 255/40-17 and a 265/35-18 is about 1% - which may not be something you notice.
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Old 01-02-2007, 01:35 PM   #3
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Thanks racer d. So in essence, what you're saying is the tradeoff is very little in acceleration, for a gain in grip.
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Old 01-02-2007, 02:47 PM   #4
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In the grand scheme of things, the difference in acceleration is negligible. You will, however, notice the change in ride quality, especially if the 18s you choose are a lot heavier. People say the change in rotational mass adds greatly to the effect of the extra weight. The math there is correct. Still, when it really comes down to it, the change in the butt dyno is really small IME.
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Old 01-02-2007, 11:01 PM   #5
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It mostly depends on the weight of the wheels. If the 18's are the same weight as the 17's then there would be very little difference. Since the rubber is thinner on the 18's the applied force to the ground would be almost the same bcecause the overall height of the wheel/tire is comparable to the 17's. As mentioned earlier, the difference in rotational mass would be neglible with such a small change in size.
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Old 01-02-2007, 11:51 PM   #6
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I'm with Adam, it is all about unsprung and rotational weight. I put some heavy 19s on my C55 and noticed the difference immediately. The car felt a bit slower to respond off-idle and low RPMs and a bit heavier in steering, too. Overall, less nimble, not that a C55 is the most nimble car to begin with.

Good luck in your decision.

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Old 01-03-2007, 12:00 AM   #7
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I say go the other way and go get some magnesium 16's with super light tires. You could have a sweet 30 LB per wheel and tire combo. lol
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Old 01-03-2007, 04:15 AM   #8
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Quote:
Originally Posted by boggtown
I say go the other way and go get some magnesium 16's with super light tires. You could have a sweet 30 LB per wheel and tire combo. lol
LOL, someone else told me that 16s would prob. make car marginally faster because of less rolling resistance. And they would look good on the car too.
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