Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > Porsche 911 Technical Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
rcilurso
Guest
 
Posts: n/a
Post Calculating tire size (long winded)

Ok, this may be common knowlege but I have had some interest in the topic so I'll babble on and share what I know. In my very humble opinion the best tire size for a 9" Fuch on the back of a non-turbo car is a 245/45/16. I like the way it looks and feels nuff said. This will apply to all preferences of size so hang in there with me. I know most of you are like me and look is almost as important as performance so we would want the front tire to compliment or look like the rear. You know what I mean, I've seen tons of P cars with nice rears and the big tall 70's looking front tires. Here comes the math part...

245 is the width in mm, so ya divide that by 25.4 to get the inches... 9.65" The second number is the aspect ratio or what percent of the width is the sidewall. 9.65 divided by .45 gives you a sidewall of 4.34. 4.34 times two plus the rim size (16) gives you the overall size of 24.68"

My problem was the fronts were too wide for the rim so they pinched up making the sidewall larger than it should have been and the tire too tall. So, to match the rear width I would want something .65" wider than the rim or 7.65" A 195 series tire gives us 7.68" a 205 series gives us 8.07" The 195 is closest. Now I know I need a 195 series tire but what aspect? The sidewall on the rear is 4.34. A 195/55 gives me a sidewall of 4.22 a 195/60 gives me 4.61. The sidewall should be shorter in the front than in the rear if it can't be equal(my opinion) so 55 it is. 4.22 times 2, plus the rim size of 16" gives me an overall of 24.44", real close to the 24.68" in the rear. At least it's not taller like it is now. So, to match the 245/45/16 on a 9" wheel I need a 195/55/16 on a 7" wheel. DISCLAIMER: I do understand the performance aspects of wider in the front and sidewall flex at the track and bla bla bla, this is just a visually pleasing exercise, and again, my rotten opinion of what is visually pleasing. Porsche obviously thought different cause I think the in stock form they had 245/45/16 rear and 205/55/16 front. Practice the math and you'll see the fronts are taller than the backs
Hope somebody can figure out what the hell I'm saying and find some use/interest in it.

Old 03-18-2001, 07:57 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
RarlyL8
Guest
 
Posts: n/a
Post

There are a couple of problems.
One is that a 7 inch wheel is not 7 inches wide. Rule of thumb is to add 3/4 inch to the advertised width. The advertised width is measured from bead to bead.
Another problem is that real world tire measurements vary from manufacture to manufacturer. An extreme example of this is two 345/35-15s I have run. The one from Yokohama was a full 3/4 inch wider tham the one from Pirrelli. This discrepancy required fender modification to cure a slight rub.
Assuming you will be running the same manufacturers tires front and rear you use a mounted tire as your base for all measurements and go from there. The math is right on, and the only thing you really have to go with. Just remember to add in a "real world" fudge factor for rubbing.
Old 03-18-2001, 08:17 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
rcilurso
Guest
 
Posts: n/a
Post

Agree 100% I have seen tons of discrepencies, but this will get ya pretty close. 345?!? good god
Old 03-18-2001, 08:20 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
89911
Guest
 
Posts: n/a
Post

Stock was 225/45 on the back.
Old 03-18-2001, 09:17 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Early_S_Man
Guest
 
Posts: n/a
RJ ... good thing your calculator/computer works better than your description ... multiply aspect ratio by section width! You obviously did the math right to get 24.68" ...

And, tires work on a wide range of rim widths! I used 235/60VR-15 Pirelli CN-36s on 15 x 6 Fuchs ... for years, on the back of my car with no rubbing, and no abnormal wear, either!

Some 225s work at the front, others have to be limited to 205s due to rubbing!

------------------
Warren Hall
1973 911S Targa
Old 03-18-2001, 09:33 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
rcilurso
Guest
 
Posts: n/a
Post

Yeah I know the tires will work on a variety of rim sizes. Problem I ran into was the look on a 7". The 7" pinched the particular size tire and made it look funny compared to what was on the rear. I was discussing this with another member, we were trying to get a front tire size that would "look" like a known rear, that is why I was babbling and comparing rim widths to the actual tire width and so on. As you know a 245 tire "sits" a certain way on a 9" rim. We were trying to duplicate that posture on a 7" front looking at widths and sidewalls and all that.

------------------
rj cilurso
87 911 targa
Old 03-18-2001, 10:11 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
 
Bill Verburg
Guest
 
Posts: n/a
Post

Instead of guessing, why not go to the manufacturers sites. They provide all the sizing info you could possibly need.
http://www.bridgestone-usa.com/
http://pirelli.pearlcomm.com/pneumatici/index.htm
etc.
Old 03-19-2001, 03:21 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
ClayMcguill
Guest
 
Posts: n/a
Arrow



Also, the Tirerack has very detailed tire specs for most tires-just choose a size, manufacturer, or type, then click on "specs" for the tires shown.



Handy.
Old 03-20-2001, 06:46 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
stlrj
Guest
 
Posts: n/a
Post

I found this calculator helpful:


http://www.miata.net/garage/tirecalc.html

Cheers,

Joe Garcia
Old 03-20-2001, 08:48 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
rattlsnak
Guest
 
Posts: n/a
Post

I'm not sure how you guys do this, but when my SC had the stock flares on it, a nine inch rim with 245/45/16 would NOT fit without rubbing. Of course, now that its flared I'm running 345/40/17'S PROBLEM SOLVED!
Old 03-20-2001, 08:56 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #10 (permalink)
RarlyL8
Guest
 
Posts: n/a
Post

Who makes a 345 for a 17 inch rim? I thought 335 was as big as it got so far. You got current pix?

Old 03-20-2001, 09:10 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #11 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 12:22 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2025, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2025 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.