Let's start with me saying I'm not a novice with cars in general and Porsches especially. I've seen a lot of threads here about what tire sizes to run and things that can mess up your diffs. I'm looking for specific opinions from people that know these cars well.

I have 235/40r19 Hankooks on the front and 305/35r19 on the rear and it's time to replace the rears at the usual interval of the fronts are good and the rears are gone. I have 305/35 on the rears. What are the thoughts on going to a 315/30-19 on the rears for the next set? I have an '04 Turbo X50 and I'd like more grip. The Hankooks are obviously what I'm going to stick with to match tread patterns and compound. Looking at the sidewall versus the rim width I can do another 10mm without any trouble.

 

Based on your current setup I assume you are not lowered, as the well over 27" diameter in your current rears would likely cause rubbing issues if you were. The 315/30's are a great choice and an almost identical match to your front 235/40's as far as diameter (about 1" less than your current rears) and revs per mile are concerned, so your differential will love you for it.

IMO I think the 10mm additional width will look better as well! Obviously you have considered spacers (or a wider one) may be necessary. Here is a pretty cool link I found for this stuff as I recently went to 19's from stock hollows: http://www.1010tires.com/TireSizeCal...?action=submit.

Good Luck!

 

Hi all. I repeat this every so often:

danger danger. ... Never, never use any online generic
'tire size calculator' if you are dealing with close tolerances, such
as trying to prevent rubbing or if you need identical front and rear
diameters like we do. They all mean well, and are simply designed
according to what the tire size nomenclature is *supposed* to
mean, but in the real world there is a wide range of variance
between what a tire is designated as and what it's real dimensions
are. For the real facts you have to go to the tire manufacturers
spec sheet. I have seen two different 245/45-16 tires differ in
diameter by 1/2 inch *from the same manufacturer*! They were
just different model tires. Recently one tire manufacturer stopped
making it's 305/30-18. Now they list a 315/30-18. *It's the same
tire*! They just decided that the actual tire dimensions were closer
to the 315 meaning, so after 6 years they just changed what was
printed on the sidewall.

To let you know how poor a ballpark answer they provide. Go to
tirerack.com and look at the spec sheets for 225-40-18 tires, and
you will find diameters varying between 24.8" and 25.5". The latter
value is taller than the average 235-40-18!

225-40-18 tires:

Hoosier A6 - 24.8"
Bridgestone expedia S-01 - 24.9"
...
Michelin Pilot Sport - 25.3"
Dunlop Sp Sport 01 - 25.5"

Now look at the supposedly larger 235-40-18 tires, eg:

Dunlop Direzza Z1 - 25.1"

I'm saying that however helpful these generic formulas are trying
to be, they are quite easily capable of being way wrong, precisely
when it matters the most to be right.
Joe
__________________

 

And Scott, there will be no real harm in going to a different brand for
the rear if you want. At this point, the time-and-heat hardening that
your front tires will have gone through will in effect make them
somewhat different than any new one you get, as well as whatever
tread depth you've lost by now. The tread pattern is also irrelevant
front-to-back. As long as you have enough tread depth in front to
evacuate water, you're safe in the rain the new rear ones will have
deeper tread so they will be OK.
It might feel like you would be wearing one brown sock and one black
one, but in truth you will be able to run and stop just as well. There is
no hidden alchemy between the front and back that could cause any
kind of 'drug-combo' reaction. In fact, you can (on purpose or not) do
much more to change the amount and type of stick you have in the
front or rear by changing alignment, pressures, damping, swaybar and/or
spring rates.
So stick to the same real as-spec'ed diameters (as close as possible)
and watch out for rubbing, and you'll be fine.
HTH,
Joe

 

Thanks for the opinions. Yes, I'm not lowered. I can't go lower with the areas I drive though I'd love to go to an X73 setup or something similar. I have a MA Shaw GT2 front bumper with carbon fiber lip and I already scrape enough places without being lowered. If I drop the car it would honestly be almost undriveable.

 

Wow that's a lot of info Joe thanks so much.

I have an extended warranty on the car so I'm looking to stick with the Hankooks just in case something goes wrong. I don't want them pointing to the tires and denying a major claim. I've already had $10k in repair work required in the last year and a half so I'm a bit gun shy. Thankfully with the extended warranty I only paid $500 for everything. The alternator went, I had an oil leak from an O-ring requiring the engine to be dropped, and the head unit went. Now I have a flashing CEL issue that I can't figure out. It's shown up twice but turning the car off/on makes it go away. Yes, I'm fully aware how bad a flashing CEL is but I believe it's tied to a bad ABS sensor and the car runs perfect, even when it happens. Of course I shut it off immediately when it happens but like I said, it's gone as soon as I restart. Both times were at 120+ mph where the right front wheel hopped on some rough pavement which is why I point to the ABS sensor.

Thanks again Joe for the excellent advice. So just to summarize, you'd say yes to going to the 315/30 on the rears for the next set?

 

If the rolling diameter of the 315/30 is close to what you run in front, OK, but if
you want more grip, rather than looks or what, you can most improve your grip
by choosing one of the ultra-summer tires, such as the Yokohama Advan Neova
or the HanKook Ventus R-S3 (if you're already using those, nevermind).

 

where did you see a 315/30/19 tire?
I know most guys use a 315/25/19 and thought a 315/30/19 didnt even exist...

 

I'm using the Ventus V12. The grip is lower than a lot of the better options, but it's still darn good. For public road driving it gives as much grip as you can really use. I drive my cars very hard and have been through just about every major brand of performance tire so I have a bit of experience, probably 200k miles or so total across a couple of 911s and a few other high performance 400 hp cars.

Yeah unfortunately I was wrong. I was told they existed but they don't. I just ended up buying another set of 305/35/19s. I was wrong, the current tires on the rear are 305/35/19 not 305/35.

 

The current consensus among the SCCA street tire racing classes I am involved with,
based on actual race times, is that the R-S3 is currently the best. In your OP you said
you wanted more grip, so if that's still true, the R-S3 will do that for you, being
significantly better for traction than the less performant V12:

Re V12:
"The Ventus V12 evo K110 is Hankook's Max Performance Summer tire developed for the drivers of sports cars, sporty coupes and high-performance sedans. The Ventus V12 evo is designed to combine comfort and control in dry and wet road conditions."

Re R-S3:
"The Ventus R-S3 is Hankook's Extreme Performance Summer tire developed for serious sports car, sports coupe and performance sedan enthusiasts looking for a race-minded street tire that can also meet the demands of autocross, drifting and track events. The Ventus R-S3 is designed to reduce the performance gap between street and track tires by significantly improving dry traction."

The difference in tread width that you are considering will make a small change in
traction, but that would be dwarfed by upgrading the tires themselves, and would
avoid any concerns about new rubbing/clearance problems. Just look at the tread
patterns of the two tires, and it becomes obvious that the R-S3 is going to put a
lot more rubber on the ground. The V12 is a mud-and-snow tire in comparison.

HTH,
Joe

 

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I am looking to get a new set of tires for 19" wheels for a C4S. Will I run into issues putting 235/35 on the front and 305/30 on the rear? Do I need to go with a 25 side wall on the rears? Will I run into issues because of the 4-wheel drive? Currently on stock 18s and looking into getting 19s.

 

I have a 2007 C4S and have been looking into tires for a couple months. I believe you should be fine with 305/30s in the rear, that's what i'm looking for. I don't even know if a 25 aspect ratio tire is made from looking around.

Unfortunately for me, i have my eyes on RE-11's, and there are no rear 305/30/R19's anywhere around me here in CA. I've been trying to get them on my car for near a month, and had to pass on a DE event last weekend because i am worried about my current tires surviving through a full weekend of racing. Tirerack estimates a 5/24 delivery, my preferred local guy can't even give me a date. I went so far as to call Bridgestone in TN today, and they said they couldn't give a date for delivery either. He estimated shipment to their Ontario, CA warehouse in a couple weeks.

In the meanwhile, i'm seriously debating PSS's...

 

I've been running Conti 315/25/19r with 235/35/19f. The tire manufacturer's spec sheet showed these to be extremely close to the stock 18" tire diameter and revolutions per mile specs.