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Rim/Tire math help

Picked up rims by chance and would like some advice from the ones that know these wide-ass cars.

My “Old” rims/wheels completely filled the tire well to the point of paper couldn’t fit between the tire and fender, but the
tires never rubbed either.

I would like the “New” rims/tires fill the well like my "Old" setup did!

My current line of thought is to replace the wheel studs with 100mm and go with 2” spacers, this would put the outer rim lip
to fender distance to within 3/8” of the “Old” setup and then let a wider tire fill the distance to the fender.

So my questions are:
Are my math and direction sound, anything I missed?
What tires both mfg and size would you recommend for street/sport in a warm climate, no DE’s?

Any advice before I pull $$$ chain would be appreciated

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2010 - 997 PDK, Black on Black, Daily driver.
1987 - 930 Grand Prix White, Not looking for crazy HP, just harmony!
Old 01-12-2008, 04:47 PM
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Crotchety Old Bastard
 
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The math sais the difference in frontspacing is 1.25" if you wish the rim face to be flush as the previus wheel.
However, if you are going from 18" to 19" and adding tires having a tread to rim ratio greater than previous things will change.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-13-2008, 05:25 AM
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How much of the 1.25 should I eliminate with spacer and what tire (width and height) would anyone recommend??

Keeping in mind the tire size is going from 17" to 18"

Quote:
Originally Posted by RarlyL8 View Post
The math sais the difference in frontspacing is 1.25"
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2010 - 997 PDK, Black on Black, Daily driver.
1987 - 930 Grand Prix White, Not looking for crazy HP, just harmony!
Old 01-13-2008, 07:12 AM
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Crotchety Old Bastard
 
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315/30-18 is a given IMHO.

275/40-17 on a 10" rim = -0.1" of pucker, your tire is streched on that rim.
315/30-18 on an 11" rim = +0.2" of pucker, tire is not streched.
Difference = 0.3"
Current spacing = 1 1/8"
Same offset with new wheels = 1 1/4", add current spacing = 2 3/8"
Subtract for new tire pucker = 2 2/25"
2" spacers should put you back to where you are now - just like you said in the original post.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-13-2008, 01:21 PM
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Thanks Brian, that was exactly the advice I was looking for!
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Old 01-13-2008, 07:00 PM
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That 11.0 inch rim in the pic looks to me like a 10.0 inch rim. IF you actually measured lip to lip and got 11.0 then it is a 10.0. You sure you want to put a 315 on a 10.0?

Cheers
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Old 01-13-2008, 07:23 PM
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Here is what I am talking about.....




Cheers
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Old 01-13-2008, 07:25 PM
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Crotchety Old Bastard
 
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My math was based on a total width of 11" and 12" respectively.
I assumed the rims were known to be 10" and 11". If the rims are actually 9" and 10" the math will change but the outcome sould not due to an equal subtraction of 1" from both. However, 315s will not fit on a 10" rim.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-14-2008, 05:26 AM
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Quote:
Originally Posted by Jeff Alton View Post
That 11.0 inch rim in the pic looks to me like a 10.0 inch rim. IF you actually measured lip to lip and got 11.0 then it is a 10.0. You sure you want to put a 315 on a 10.0?

Cheers
Jeff, forgetting about what width the rim actually is, here is my naďve take on widths (I am no expert on this subject, this is for my own edification)

The Old rim is 10 in lip-to-lip = 254 mm and the New rim is 11 in = 279.4 mm.

Since the rim lip is the last contact with the tire before the tread surface.
Isn’t the goal to cover this width with the widest tread and minimize the “Pucker” RarlyL8 speaks of?

So wouldn't a 315 mm tread on a 279.4 mm lip-to-lip rim achieve this goal?

Am I missing something here?
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1987 - 930 Grand Prix White, Not looking for crazy HP, just harmony!
Old 01-14-2008, 07:30 AM
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Actually, no.
The best handling, which is minimal but predictable sidewall flex, comes with a sidewall width equal to the total rim width. So a 10" rim, which is actually 11" wide will best fit a 280mm tire. A 315mm tire will have 17.5mm (0.7") of pucker which is too much on a low-profile tire.

Are you measuring inside the bead or outside? What size is stamped on the wheel?
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-14-2008, 08:36 AM
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Quote:
Originally Posted by RarlyL8 View Post
Are you measuring inside the bead or outside? What size is stamped on the wheel?
Jeff was correct in the size, it is stamped 18x10 on the back.

I am measuring outer lip-to-lip
When I bought the rims, they had very worn out 315's on them.
They came off of a wide-body conversion.


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1987 - 930 Grand Prix White, Not looking for crazy HP, just harmony!
Old 01-14-2008, 09:27 AM
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Crotchety Old Bastard
 
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How did those worn out 315s look on that wheel? What brand were they?
Another thought is that actual tire sizes vary by manufacturer.
Pirelli is notorious for undersizing their tires. If you can stand the $800 price tag for a set you might be able to fit 315s on those wheels and have them look OK. If not then go with 295/30/18s. What are you running up front?
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-14-2008, 10:17 AM
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Unfortunately I don’t remember how they looked, bought the wheels and pulled the tires last fall.
My plan is to go with 235’s up front on a 9 in rim (or 8 now), this was on my old setup and looked and cleared everything just fine.
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2010 - 997 PDK, Black on Black, Daily driver.
1987 - 930 Grand Prix White, Not looking for crazy HP, just harmony!
Old 01-14-2008, 10:29 AM
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235s on 8s and 285s on 10s give a similar PF.
255s on 9s and 315s on 11s give a similar PF.
Be mindful of tire hieght as well. An aspect ratio increase of 5%, such as going from a 35 to a 40 series is 1" for a 235!
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-14-2008, 01:15 PM
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Thanks Brian and Jeff for tolerating all the basic questions I was asking, learned a lot on this thread.
I really appreciate your help!
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2010 - 997 PDK, Black on Black, Daily driver.
1987 - 930 Grand Prix White, Not looking for crazy HP, just harmony!
Old 01-14-2008, 04:20 PM
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Your questions are not basic at all. You would not beleive the number of wheels we have built to customer specs in the past only to have them say "these are wider than I ordered!"

That is why I had that pic handy.....

Cheers
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Old 01-14-2008, 05:11 PM
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Found this handy link for calculating tire size, posting for posterity ->Click here
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2010 - 997 PDK, Black on Black, Daily driver.
1987 - 930 Grand Prix White, Not looking for crazy HP, just harmony!
Old 01-17-2008, 10:35 AM
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Handy tool!
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 01-17-2008, 12:47 PM
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Thanks for the link, Cl8ton. I use this calculator when I am trying to visualize the difference between what I have and what I want will look like.
http://www.rims-n-tires.com/rt_specs.jsp
Old 01-17-2008, 03:44 PM
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