Tire size recommendation for 18x10 rear
12-18-2016, 12:41 AM
#31
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Was not looking to be argumentative, just helpful to the OP.
265 is a stock width and 285 is .78 of an inch wider than stock.
Stock F/R stagger (all things equal on suspension, not to be assumed on a 20 year old car) has built-in understeer.
3/4 inch more rear tire adds to the understeer.
If you are driving n the street, you really should not be driving so fast as to induce any understeer (in my personal opinion). If you drive in snow, sand, rain or on the track - that's different and outside of this discussion.
A 993 will be perfectly safe, driveable, and enjoyable on a 225/285 setup in street usage. Not ideal, but I don't think discarding new front 225 tires makes any sense. 235 would be better, 275 good too. Depends what is available and not worth the drama (again, just my opinion).
Cheers
Matt
265 is a stock width and 285 is .78 of an inch wider than stock.
Stock F/R stagger (all things equal on suspension, not to be assumed on a 20 year old car) has built-in understeer.
3/4 inch more rear tire adds to the understeer.
If you are driving n the street, you really should not be driving so fast as to induce any understeer (in my personal opinion). If you drive in snow, sand, rain or on the track - that's different and outside of this discussion.
A 993 will be perfectly safe, driveable, and enjoyable on a 225/285 setup in street usage. Not ideal, but I don't think discarding new front 225 tires makes any sense. 235 would be better, 275 good too. Depends what is available and not worth the drama (again, just my opinion).
Cheers
Matt
We can discuss street usage. I drive like a grandma on the street. And agree that really discussing any "steer" for street driving is a fairly moot. My point, again, is that the vector of "improvements" by CorrdoBrit is wrong. I also think that 30-ratio tire is not a good idea for a street car. Especially a 30-ration tire with a known floppy sidewall. I have hard time understanding that specific combination - very low profile with soft wall. Which was necessitated by 285 choice. That's all I'm saying, honestly
12-18-2016, 12:45 AM
#32
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I know the NB 993's call for a narrower rear - which I assumed was simply because that was the max space (comfortably) available on a NB in stock trim. But given that the C4S has the same engine as a C2/C4/C2S, why would Porsche have specified 285's in the rear?
12-18-2016, 01:14 AM
#33
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Matt, these might all be semantics, but... my point is, why make car handle worse when you can leave it alone, if not make it better.
We can discuss street usage. I drive like a grandma on the street. And agree that really discussing any "steer" for street driving is a fairly moot. My point, again, is that the vector of "improvements" by CorrdoBrit is wrong. I also think that 30-ratio tire is not a good idea for a street car. Especially a 30-ration tire with a known floppy sidewall. I have hard time understanding that specific combination - very low profile with soft wall. Which was necessitated by 285 choice. That's all I'm saying, honestly
We can discuss street usage. I drive like a grandma on the street. And agree that really discussing any "steer" for street driving is a fairly moot. My point, again, is that the vector of "improvements" by CorrdoBrit is wrong. I also think that 30-ratio tire is not a good idea for a street car. Especially a 30-ration tire with a known floppy sidewall. I have hard time understanding that specific combination - very low profile with soft wall. Which was necessitated by 285 choice. That's all I'm saying, honestly
I don't understand why this should be a problem on a NB vs the C2S WB. Apart from wider rear wheel fender flares the cars to all intents and purposes are identical.
12-18-2016, 02:06 AM
#34
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Are these opinions based on personal experience or assumptions? In the limited amount of time I've been running this combination I've not noticed any ill effects, but as I've said I'm not pushing the car to the limit on the street. In fact I feel acceleration has noticeably improved, with the 30 profile tire compensating for the high gearing in US spec cars.
I don't understand why this should be a problem on a NB vs the C2S WB. Apart from wider rear wheel fender flares the cars to all intents and purposes are identical.
I don't understand why this should be a problem on a NB vs the C2S WB. Apart from wider rear wheel fender flares the cars to all intents and purposes are identical.
Maybe you can explain your line of thinking with going to 285 and 30 ratio. Mind you, it's your car, but it's a curious and odd direction to me.
12-18-2016, 07:01 PM
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12-18-2016, 08:31 PM
#36
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Choices. I'm talking about choices. Why chose 285 and force yourself to go to 30 on an NB car?
But, perhaps you guys are right. A soft sidewall tire with 30 ratio is the perfect choice for the rear tire of a street 993.
But, perhaps you guys are right. A soft sidewall tire with 30 ratio is the perfect choice for the rear tire of a street 993.
12-22-2016, 03:21 AM
#38
This thread is frustrating. Google tire size aspect ratios and you will realize a "30" sidewall on a 285 is the same as a "35" on a 265. Given the same size rim it's just a percentage of the tires width and in result overall diameter and sidewall will be the same. Maybe I missed something and I know many people who posted understand this but even tire guys at tire shops alot of times don't have their head around this equation. Good luck
12-22-2016, 04:19 AM
#39
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This thread is frustrating. Google tire size aspect ratios and you will realize a "30" sidewall on a 285 is the same as a "35" on a 265. Given the same size rim it's just a percentage of the tires width and in result overall diameter and sidewall will be the same. Maybe I missed something and I know many people who posted understand this but even tire guys at tire shops alot of times don't have their head around this equation. Good luck
You are definitely missing something. Where did you get the same overall diameter? Is it that hard to multiply 285 by .3 and 265 by .35 and see almost 10% difference in sidewall? And, BTW, the size of the rim has nothing to do with it.
12-22-2016, 10:05 AM
#41
I'm not sure you can go by straight math; the tire dimensions depend on individual tire model specifications.
As an example below with a 10-inch wide x 18-inch diameter wheel mounted with either Pirelli P-Zero Rosso N4 Porsche spec
265/35-18; 10.8-in overall section width; 3.7-in sidewall height; 25.4-in overall diameter
285/30-18; 11.4-in overall section width; 3.6-in sidewall height; 25.1-in overall diameter.
OR Bridgestone S-02A N3 Porsche spec
265/35-18; 11.3-in overall section width; 3.5-in sidewall height; 25.0-in overall diameter.
285/30-18; 11.3-in overall section width; 3.5-in sidewall height; 24.9-in overall diameter.
In the examples above the sidewall heights were derived from the overall tire diameters listed in Tire Rack's specifications for each tire model.
The overall section widths were derived from Tire Rack's section widths when mounted on each tire manufacturer's standard specification rim width, + 0.2-inch tire section width added with each 0.5-inch increase in rim width (refer to Tire Rack's Tire Tech section).
In these specific examples the tire sidewall heights are not very different, but this might not be the case with other tire models when comparing 265/35 and 285/30-18 tire sizes.
The Bridgestone S-02A N3 Porsche-spec comparison is an unusual case where the 265/35-18 and 285/30-18 dimensions are the same when mounted on a 10 x 18 wheel.
Since the sidewall heights and overall section widths are based on specification data, one would have to verify them when each tire size is actually mounted on the 10 x 18 wheel.
Use the above to help you estimate possible differences each size might have, when considering them for your application.
As an example below with a 10-inch wide x 18-inch diameter wheel mounted with either Pirelli P-Zero Rosso N4 Porsche spec
265/35-18; 10.8-in overall section width; 3.7-in sidewall height; 25.4-in overall diameter
285/30-18; 11.4-in overall section width; 3.6-in sidewall height; 25.1-in overall diameter.
OR Bridgestone S-02A N3 Porsche spec
265/35-18; 11.3-in overall section width; 3.5-in sidewall height; 25.0-in overall diameter.
285/30-18; 11.3-in overall section width; 3.5-in sidewall height; 24.9-in overall diameter.
In the examples above the sidewall heights were derived from the overall tire diameters listed in Tire Rack's specifications for each tire model.
The overall section widths were derived from Tire Rack's section widths when mounted on each tire manufacturer's standard specification rim width, + 0.2-inch tire section width added with each 0.5-inch increase in rim width (refer to Tire Rack's Tire Tech section).
In these specific examples the tire sidewall heights are not very different, but this might not be the case with other tire models when comparing 265/35 and 285/30-18 tire sizes.
The Bridgestone S-02A N3 Porsche-spec comparison is an unusual case where the 265/35-18 and 285/30-18 dimensions are the same when mounted on a 10 x 18 wheel.
Since the sidewall heights and overall section widths are based on specification data, one would have to verify them when each tire size is actually mounted on the 10 x 18 wheel.
Use the above to help you estimate possible differences each size might have, when considering them for your application.
choosing a tire at random, say a BS RE-11
nominal size 285/30 x18
if mounted on a 10, OD is 24.8" s/w is 11.4", t/w is 10.5", loaded rolling OD at 34psi is 23.98"
if mounted on a 9.5, OD is a tad more than 24.8", s/w is 11.2", t/w is 10.5" loaded rolling OD @ 34psi is a tad over 23.98"
In my experience the lower the car the more work to fit a 285/30 x18 on a 10ET65 wheel to a 993
that said the factory did a huge amount of tire testing when they developed the 993Cup and RSR, both came w/ 245/35 & 285/30 x18 tires on 8.5 & 10 as delivered to customers, the Cups raced that way the RSR's got bigger rubber & wheels
12-22-2016, 10:58 AM
#42
Do you want to check your math there a little, cowboy?
You are definitely missing something. Where did you get the same overall diameter? Is it that hard to multiply 285 by .3 and 265 by .35 and see almost 10% difference in sidewall? And, BTW, the size of the rim has nothing to do with it.
You are definitely missing something. Where did you get the same overall diameter? Is it that hard to multiply 285 by .3 and 265 by .35 and see almost 10% difference in sidewall? And, BTW, the size of the rim has nothing to do with it.
12-22-2016, 12:44 PM
#43
Race Car
^^ Bill has the Data!
My experience based on running everything from 265/35r18 through to 295/30r18 and all in between on either a 10 or 10.5 in the rear and running from a 235/40r18 to a 255/35r18 on 9.5 to 9 on the front.
Additionally, All my experience has been hundreds of track laps, with GPS data, with many combinations of the above, from high performance street tires; Direzza, RE71's, to Hoosiers, R888 and NT01 down to 300 wear rating summer tires. Many of these tire changes have also happened without major tweeks to the suspension and setup, and keeping the same alignment save for the hoosiers.
Best Balance between Under and over steer (Equal Winners):
235/40r18 Front on 8.5 and 265/35r18 on 10
245/35r18 Front on 8.5 and 285/30r18 on 10.5
These 2 combinations provided the best balance overall with the edge going to the 285 mainly because of the decreased diameter giving a little better pull out of corners (GPS verified), Hoosier R6 and R888 are both 24.8" Tall, the R7 is now 24.9, The 265's I have run are 25.3 or 25.4 so just around 1/2" taller
Second Place:
245/35r18 Front on 8.5 and 295/30r18 on 10.5 rear - Good balance, with the slightly taller rear tire didn't have the same eagerness out of the corners, but balance was excellent.
Least favourite was the 245/40r18 on 8.5 and 275/35r18 on 10 and a 255/35r18 on 8.5 and 285/30r18 on 10. These just didn't provide the same balance for me overall, the cornering g's with similar tires was lower, just not my cup of tea.
Just my 2c worth
My experience based on running everything from 265/35r18 through to 295/30r18 and all in between on either a 10 or 10.5 in the rear and running from a 235/40r18 to a 255/35r18 on 9.5 to 9 on the front.
Additionally, All my experience has been hundreds of track laps, with GPS data, with many combinations of the above, from high performance street tires; Direzza, RE71's, to Hoosiers, R888 and NT01 down to 300 wear rating summer tires. Many of these tire changes have also happened without major tweeks to the suspension and setup, and keeping the same alignment save for the hoosiers.
Best Balance between Under and over steer (Equal Winners):
235/40r18 Front on 8.5 and 265/35r18 on 10
245/35r18 Front on 8.5 and 285/30r18 on 10.5
These 2 combinations provided the best balance overall with the edge going to the 285 mainly because of the decreased diameter giving a little better pull out of corners (GPS verified), Hoosier R6 and R888 are both 24.8" Tall, the R7 is now 24.9, The 265's I have run are 25.3 or 25.4 so just around 1/2" taller
Second Place:
245/35r18 Front on 8.5 and 295/30r18 on 10.5 rear - Good balance, with the slightly taller rear tire didn't have the same eagerness out of the corners, but balance was excellent.
Least favourite was the 245/40r18 on 8.5 and 275/35r18 on 10 and a 255/35r18 on 8.5 and 285/30r18 on 10. These just didn't provide the same balance for me overall, the cornering g's with similar tires was lower, just not my cup of tea.
Just my 2c worth
12-22-2016, 01:02 PM
#44
^^ Bill has the Data!
My experience based on running everything from 265/35r18 through to 295/30r18 and all in between on either a 10 or 10.5 in the rear and running from a 235/40r18 to a 255/35r18 on 9.5 to 9 on the front.
Additionally, All my experience has been hundreds of track laps, with GPS data, with many combinations of the above, from high performance street tires; Direzza, RE71's, to Hoosiers, R888 and NT01 down to 300 wear rating summer tires. Many of these tire changes have also happened without major tweeks to the suspension and setup, and keeping the same alignment save for the hoosiers.
Best Balance between Under and over steer (Equal Winners):
235/40r18 Front on 8.5 and 265/35r18 on 10
245/35r18 Front on 8.5 and 285/30r18 on 10.5
These 2 combinations provided the best balance overall with the edge going to the 285 mainly because of the decreased diameter giving a little better pull out of corners (GPS verified), Hoosier R6 and R888 are both 24.8" Tall, the R7 is now 24.9, The 265's I have run are 25.3 or 25.4 so just around 1/2" taller
Second Place:
245/35r18 Front on 8.5 and 295/30r18 on 10.5 rear - Good balance, with the slightly taller rear tire didn't have the same eagerness out of the corners, but balance was excellent.
Least favourite was the 245/40r18 on 8.5 and 275/35r18 on 10 and a 255/35r18 on 8.5 and 285/30r18 on 10. These just didn't provide the same balance for me overall, the cornering g's with similar tires was lower, just not my cup of tea.
Just my 2c worth
My experience based on running everything from 265/35r18 through to 295/30r18 and all in between on either a 10 or 10.5 in the rear and running from a 235/40r18 to a 255/35r18 on 9.5 to 9 on the front.
Additionally, All my experience has been hundreds of track laps, with GPS data, with many combinations of the above, from high performance street tires; Direzza, RE71's, to Hoosiers, R888 and NT01 down to 300 wear rating summer tires. Many of these tire changes have also happened without major tweeks to the suspension and setup, and keeping the same alignment save for the hoosiers.
Best Balance between Under and over steer (Equal Winners):
235/40r18 Front on 8.5 and 265/35r18 on 10
245/35r18 Front on 8.5 and 285/30r18 on 10.5
These 2 combinations provided the best balance overall with the edge going to the 285 mainly because of the decreased diameter giving a little better pull out of corners (GPS verified), Hoosier R6 and R888 are both 24.8" Tall, the R7 is now 24.9, The 265's I have run are 25.3 or 25.4 so just around 1/2" taller
Second Place:
245/35r18 Front on 8.5 and 295/30r18 on 10.5 rear - Good balance, with the slightly taller rear tire didn't have the same eagerness out of the corners, but balance was excellent.
Least favourite was the 245/40r18 on 8.5 and 275/35r18 on 10 and a 255/35r18 on 8.5 and 285/30r18 on 10. These just didn't provide the same balance for me overall, the cornering g's with similar tires was lower, just not my cup of tea.
Just my 2c worth
a friend w/ a 993 setup similar to mine likes MPSC on Speedline 8ET52 & 10ET65 x18, so he pretty much needs to use 235/40 & 285/30
On the same wheels I wanted to use RE71R so I pretty much need to use 225/40 & 265/35.
Another set of wheels 8.5ET54 & 10ET65 I want NT01 so need to use 235/40 & 275/40
If I wanted to use 245/35 & 285/30(which I do) I would have to switch to Hoosier, RE11 or Kumho, each of which is unacceptable for other reasons
Wish I was brave enough to spring for a set of Trophys 10.5ET59 size
I see lots of 993n/b w/ 285/30 x18 on 10ET65, I have seen 1 other low 993 on 295/30 x18 but don't know the wheel specs
12-22-2016, 01:14 PM
#45
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