# 18's slower than 17's?



## kegbelly (Mar 23, 2006)

I've read several posts here about 17" vs. 18" wheels, but haven't seen this particular topic addressed...
Do larger wheels adversely affect straight-line acceleration of a vehicle? In this case I'm referring to the 17" wheels on the GTO vs. the optional 18's. I understand you would theoretically gain some handling and cornering advantages due to the lower profile tires on a larger wheel, but an 18" wheel is obviously larger than a 17" wheel, so it contains more steel/aluminum/whatever, which weighs more than the air in a 17" tire. A buddy of mine brought this up at work recently after he put some 19's on his Tahoe (I forget if it had 16's or 17's stock). He said it seriously slowed the vehicle down from take-off and dropped about 3 mpg off his average. He swears its because the larger wheels are so much heavier than stock due to more steel and less air/rubber. He's a pretty serious gearhead, and I don't ordinarily doubt his word. Just curious...any thoughts?

Kegbelly


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## PHOTOGOAT (Feb 17, 2006)

It would be due to the overall diameter of the tire vs. stock, and not weight. By increasing the diameter of the rolling stock, it will in effect lower the ratio in the rearend.

The newer fancy wheels are pretty light compared to the factory stuff. 

GTO 17 and 18 inch rolling stock have the same overall diameter, just a lower profile tire.

:cheers


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## gtoh (Dec 9, 2005)

A good guesstimate for the increase in wheel size is that every pound you add per wheel is like adding about 4 pounds into your car, so if you have a wheel/tire combo of 30 pounds apiece and buy 19's that weigh 40 pounds apeice, it would be like adding 160 pounds to your car, since this is all unsprung weight. This theory holds pretty true in 1/4 mile runs. I had a gtp that would run 13.3's all day and when I stuck on some heavy 18's I was looking at 13.7's. Not to mention the further out you push the weigh from the hub as you would with 18's from 17's the harder it is for your car to get them moving. That being said I would imagine the factory 18" wheels from gm are fairly light and similarly weighted to the 17's so the difference would be barely anything, but technically since the 18's weight is situated further out from the hub it should be technically slower I suppose. More than likely though driver skill would play more of a roll in this case....unless you were going from 17's to like 20's.


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## kegbelly (Mar 23, 2006)

gtoh said:


> A good guesstimate for the increase in wheel size is that every pound you add per wheel is like adding about 4 pounds into your car, so if you have a wheel/tire combo of 30 pounds apiece and buy 19's that weigh 40 pounds apeice, it would be like adding 160 pounds to your car, since this is all unsprung weight. This theory holds pretty true in 1/4 mile runs. I had a gtp that would run 13.3's all day and when I stuck on some heavy 18's I was looking at 13.7's. Not to mention the further out you push the weigh from the hub as you would with 18's from 17's the harder it is for your car to get them moving. That being said I would imagine the factory 18" wheels from gm are fairly light and similarly weighted to the 17's so the difference would be barely anything, but technically since the 18's weight is situated further out from the hub it should be technically slower I suppose. More than likely though driver skill would play more of a roll in this case....unless you were going from 17's to like 20's.


Thanks to both of you for the great info. I know my buddy said his aftermarket wheels were so heavy it was all he could do to handle putting them on by himself. They were chromed steel by the way, so I'm sure they were much heavier than stock. Sounds like both factors affected his performance: larger diameter wheels= lower effective rear end ratio, plus heavier wheels= more unsprung weight.


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## GTJoe (Jun 28, 2006)

PHOTOGOAT said:


> It would be due to the overall diameter of the tire vs. stock, and not weight. By increasing the diameter of the rolling stock, it will in effect lower the ratio in the rearend.
> 
> The newer fancy wheels are pretty light compared to the factory stuff.
> 
> ...


Not quite true( the stock equipped 17's come with 245/45 tires - 25.7" the stock equipped 18's come with 235/40- 25.4" )


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