CKY881

Hi, i'm trying to go for the low profile look on my 18x9.5 all around rims, but im not sure how big the difference is... I'm looking at 245/35/18 & 275/35/18 as opposed to 245/40/18 & 275/40/18... What is the sidewall difference for these 2? I heard ride quality for 35 profiles will be bad though. How about getting thinner tires, and staying with 40 profile? Would getting thinner tires make the sidewall smaller? Maybe a 235/40/18 & 255/40/18...

Im very confused about this ratio thing...

Spike100

^^ The metric tire sizing system represents a tire size as:
[section width (mm)] / [aspect ratio] - [rim diameter (inches)]

For example: a tire size 225/45-18 is 225mm wide, has an aspect ratio of 45 and fits an 18" rim (wheel).

The aspect ratio (the second number) is not a measurement. The aspect ratio represents the tire's sidewall height as a percentage of the tires width (section height divided by section width). In the example above, the sidewall height is 101mm (101/225=45).

The aspect ratio is important since that (along with the wheel size) determines the overall diameter of the tire allowing you to match your stock overall tire height (diameter). For example, a 225/45-18 tire's sidewall height is 101mm, and a 245/40-18 tire's sidewall height is 98mm.

So, both tires have nearly the same overall diameter.

--Spike

CKY881

Thanks Spike, so basically the thinner the tire is, the less sidewall height it will have right? Assumming the aspect ratio is the same for both in the comparasion.

DMXfan247

Great answer.. couldn't put it better myself..

CKY881

Would running 235/40/18F and 255/40/18R cause handling issues? Oversteering and understeering?

terrasmak

iTrader: (8)

Since there is still a 20 mm different in width front to back , it will stay the same as stock.

CKY881

Ive heard 245F and 275R is the optimal size and causes less understeering, but I dont want such a meaty look especially on 18"...

terrasmak

iTrader: (8)

255f and 275r is actually optimal, and on the 245/275 combo , it will increase understeer.

Eddie@Performance

iTrader: (4)

You could always run a 245 35 18 fronts and 275 35 18 rears. That has alot less sidewall but still conforms to TCS/VDC requirements in sizing. But if you are running 9.5 wide all around i sugesst 275 35 18 fronts and 275 40 18 rears. Those sizes work.

terrasmak

iTrader: (8)

+1 , and the 275/275 combo would actually be my choice.

davidv

iTrader: (2)

The difference between 35 and 40 percent is 5 percent. Sometimes I amaze myself.

CKY881

What if I go 1 size smaller, 235/35/18 & 265/35/18? I don't have VDC/TCS so that's not a problem.

Ok how about I ask this: What's the lowest profile tires I can go on 18" without sacraficing handling issues? I mean stock has 225F and 245R, as long as I don't get anything worse than that, handling shouldn't be worse than how it is right now right?



Dave I have no idea what you just said lol.

davidv

iTrader: (2)

LOL. Like Spike100 said. Sidewall height is a percentage of width.

davidv

iTrader: (2)

Speedometer will read 101.01 MPH at 100.0 MPH. That’s close to perfect.

Eddie@Performance

iTrader: (4)

I think 235 on a 9.5 would look a bit silly. The tire would be super stretched and there isnt alot of sectional width either. Unless your Z is super slammed, i found that very low pro 18" tires look very awkward on a 350Z.

CKY881

That's true, because it creates an even bigger gap eh?


If I get 245/35F & 265/35R, it wouldnt make much difference as opposed to having 275/35R right? 10mm is barely noticable.


Edit:
How do you measure the sidewall height exactly? I measured on stock 07 wheel (225/45/18) and it came out to be just about 5 cm (50mm). But when I calculated the sidewall height by multiplying the tire width by the aspect ratio (225 * 0.45), I get 101.25 mm, which = ~10 cm... The two numbers dont match up though. Am i doing something wrong?

Spike100

10mm is relatively small when comparing section widths or tread widths, significant when comparing wheel offsets, and huge when comparing section heights.

You are measuring your tire’s section height (you say sidewall height) incorrectly. Your measurement is “short.” The red bracket in your picture should extend down to the bottom of the tire.

Tire manufacturers measure the section height (what you are calling sidewall height) on a suspended wheel at maximum inflation (which is probably around 50 psi, but the tire on the Z is run at 35 psi).

And... the aspect ratio is not the section height.

Look at this link for the answers to your questions:

http://www.tirerack.com/tires/tirete...e.jsp?techid=7

Hope this is helpful,

--Spike

CKY881

So basically I am measuring the top side wall, but am forgetting the bottom sidewall? In this case, by adding the individual side walls (top & bottom 5+5 cm), it comes out to be ~10cm, which is the number I got for multiplying 225 width tire by 0.45 aspect ratio.

Please confirm it Spike, thanks

davidv

iTrader: (2)

No need to do the arithmetic yourself: http://www.dakota-truck.net/TIRECALC/tirecalc.html.

CKY881

Very helpful site, thanks Dave. So by switching from 225/45 to 245/35, odometer and speedometer will be 4.93% higher. And for the rear, switching from 245/45 to 275/35 will increase reading by 4.31%. Does the car take the average of these two when displaying the odometer and speedometer? Also, is there any way to fix this, but still keep the 35 ratio? Odometer is what concerns me more (don't want to add anymore mileage to my car than it already has).