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Discussion Starter #1 (Edited)
I do not like the standard wheels on the ST at all, so will definitely be getting new wheels immediately, probably BBS CF wheels (the same ones on the ST-R). Despite most people wanting ever larger wheels, I have been considering going with a smaller wheel. The stock setup is 235/40R18 tires on 18x8 wheels. I thought I might go with 215/50R17 tires on 17x7.5 wheels. The main reason I am thinking about this is related to ride comfort. I am almost 50 and have problems with my two lower vertebrae, so ride comfort is important to me, though I still want a car that is very fun to drive. Based on my research, the circumference of the two tires is almost identical, only a 0.2 inch difference, so speedometer readings should be near identical, showing 60.1 when actually 60. I have no intention of racing the car...just a small amount of daily driving, and a few very long trips (approximately 11 hours each direction) every year.

Any experience or ideas regarding how much difference this will make in ride comfort over expansion joints, pot holes, railroad tracks, etc?

I also thought this setup could provide a small boost in gas mileage. I theorize the mileage boost because this setup would have the tires rotating 2 less times per mile, which means all other things being equal, the engine rpm would be (slightly) lower. Since the tires have 20mm less surface contact with the road, the lower friction should help improve economy as well. Any experience or ideas regarding how much improvement I could expect in gas mileage with this setup?
 

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WEll I aint too far behind ya.. Im 45 and, yeah, lo-pro tires on gigando rims will make your teeth fall out.
I would go with the FATTEST tire on a stock rim size. and that is because I am little worrried about brake rotor clearance.

On my charger, I went 255-50-18's and I LOVE them they really suck up te vibration and still handle nicely.


take a whiff of these:

245/50ZR18 XL 104W B Nitto Invo's

nithz0.xl.jpg
 

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I have to LOL she unread this thread. When I had a Honda Civic I thought it was something when I put on a set of 15's. My Focus came with 16's and my current Focus has 18's. I have the titanium handling package. The ride is firm, not harsh.
 

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I'd expect the ST's stock ride to be relatively comfortable comparably, but also more firm than a non-performance model. I like the idea of "downgrading" to 17s to get the less edgy ride and the less expensive daily driver tires. I'd like to save the good tires for auto-x and such.
 

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Discussion Starter #6 (Edited)
And of course smaller usually means less weight as well.
That's true...each BBS CF 17 wheel is 3 pounds lighter than the 18...a total savings of 12 pounds at the wheels. Comparing Conti Extreme Contact DW tires, the 215/50R17s were 1 pound less each than the 235/40R18s, so another 4 pounds saved at the tires.

I'm feeling even more confident now that this change to 17s will not only improve ride comfort, but also increase gas mileage, and it will do it with no discernible difference in daily driving performance (though of course it may take a couple of seconds off your track lap times).
 

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Just for my info. Won't smaller tires make a difference in the speedometer reading? a smaller tire will turn faster to cover the same distance. So won't the speedometer (and odometer, fuel eff. calc, torque vectoring and anything else that depends on wheel travel distance) be compromised?
 

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Good question
 

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But what about the traction control and the torque vectoring systems, how would it impact those systems.
 

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Discussion Starter #11 (Edited)
The circumference is almost identical (only different by 0.2 inches) because the tire is taller, so the speedo will show 60.1 when actually doing 60. Keep in mind that although the wheel is smaller, the tire is bigger (taller). The taller tire (50 series compared to 40) is what provides a smoother ride. I doubt traction control or torque vectoring will be affected since the overall circumference of the wheel/tire combo is the same, well extremely close anyway.
 

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.2" difference in circumference means about a 1/16" tire outside diameter difference. This is much less than the size difference in between new tires and worn tires (as much as 1/2" diameter difference). The most it will affect would be a virtually unnoticeable difference in your speedometer reading.
 

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That's true...each BBS CF 17 wheel is 3 pounds lighter than the 18...a total savings of 12 pounds at the wheels. Comparing Conti Extreme Contact DW tires, the 215/50R17s were 1 pound less each than the 235/40R18s, so another 4 pounds saved at the tires.

I'm feeling even more confident now that this change to 17s will not only improve ride comfort, but also increase gas mileage, and it will do it with no discernible difference in daily driving performance (though of course it may take a couple of seconds off your track lap times).
More important that a mere 12-16 lbs of weight loss... it's 3-4 lbs of UNSPRUNG WEIGHT AT EACH CORNER. That is significant!
 
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