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Glenn 911

Tire size help needed - 19" Rims

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Hi,

 

I have a 2004 996 TT.  I purchased the car with 19" rims on it and I'm not sure what to do about replacement tires.  The front tires that came on it are Michelin 235/35 ZR 19, the rear tires are Azenus 275/30/ ZR 19.

With the difference between the stock recommended and the mismatch on the tires now, I'm not sure if any thought was given to the correct sizing.  I have to replace the fronts, the rears are new.  I'm

more inclined to replace all four. Does anyone have any insight as to what the best size selection would be for this ?  It's a summer car that only sees rain when the unexpected shower shows up if we

are out on the road. Does consideration have to be given to the 19" rim for height, i.e. do I have to keep a lower profile tire due to the 19" rim vs an 18" or 17" rim ?

I would like to go a little wider even if I had to put some spacers on as I'd like to enhance the look of the wide hips of the car. Would 235/40 19" up front and 295/30 19" rear work and accomplish this ?

Any thoughts would be appreciated !

 

Thanks

 

Glenn

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What is your priority? Looks or performance? If looks, just try to find a tire size with similar outside dimensions to the stock tires. The rim size doesnt matter its the O.D. Of the tire that is important. The 19" wheel will use a shorter sidewall tire compared to the 18" wheel, but the o.d. Needs to be close to the same.

If Performance is your goal, ditch the 19" wheels and go back to the 18". Thats what the car was designed for. 19" will work but because of the shorter sidewall the suspension characteristics will change a bit and the car will tend to tramline. There is no advantage to larger wheels unless you need larger brakes. In GT racing the only time they run 19" wheels is if the track is fast and they need to run more brake.

But if you just want the look of the 19" just keep the o.d.s close to stock.

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What is your priority? Looks or performance? If looks, just try to find a tire size with similar outside dimensions to the stock tires. The rim size doesnt matter its the O.D. Of the tire that is important. The 19" wheel will use a shorter sidewall tire compared to the 18" wheel, but the o.d. Needs to be close to the same.

If Performance is your goal, ditch the 19" wheels and go back to the 18". Thats what the car was designed for. 19" will work but because of the shorter sidewall the suspension characteristics will change a bit and the car will tend to tramline. There is no advantage to larger wheels unless you need larger brakes. In GT racing the only time they run 19" wheels is if the track is fast and they need to run more brake.

But if you just want the look of the 19" just keep the o.d.s close to stock.

Thanks for your response,

 

Buying the car second hand, I don't know all the mods that have been done to the car as far a larger brakes etc, I'm satisfied with the overall performance and don't plan on racing.  So based on your note, I am correct that the 235/40 19" up front and 295/30 19" rear will work,  correct ? The 19" rim size is only referring to the rim size the tire will fit, I don't have to buy lower profile tires to compensate for larger rims. I just want to be certain before I put an order in for these tires.

 

Thanks

Glenn

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There is a % difference the front and rear wheel diameter needs to fall within so the VC stays happy.  I forget but it is something like 3% (don't quote me on this % number and please research what the acceptable % difference needs to be)

 

Your proposed setup

235/40-19 = 26.4"

295/30-19 = 25.97"

% difference = 1.64%

 

My setup

245/35-19 =  25.75"

295/30-19 =  25.97"

% difference = .85%

 

You want to be as close to zero as possible. If I remember correctly, Porsche's summer and winter tires recommendations all fall under 1% in diameter difference.

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Realize that the % differences you are quoting above are completely theoretical....unless you have actually measured the rolling diameters of the tires mounted...The Porsche recommendation is <3% and If I recall it is better to bias the difference to the fronts????

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Realize that the % differences you are quoting above are completely theoretical....unless you have actually measured the rolling diameters of the tires mounted...The Porsche recommendation is <3% and If I recall it is better to bias the difference to the fronts????

Well yeah, just showing the OP which tire size of preferred. Obviously in the real world you can have fronts with less thread depth, different psi to throw off the % but as long as you are in the ballpark, you will be fine.

There were theories why the rear should be slightly smaller then front but I think those were busted.

Without looking up the exact numbers, the Porsche summer tire recommendation had a slightly smaller rear diameter, while the winter recommended sizes have a slightly smaller front. I don't think it really matters which one is smaller as long as they stay within a certain % difference.

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If the tire sizes are exactly equal, there is no shear at the VC coupling, hence minimal drag, or lock up.  If there is a front to rear bias in outer diameter of the tire, then the VC silicone fluid is shearing - and causing some low % of lock up.  Smaller rear diameter causes transfer of power to the front.  smaller front diameter causes drag on the engine at steady cruise speed.  Disclaimer, I was an engineer at a major silicone fluid manufacturer in Germany for 13 years of my career...

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If the tire sizes are exactly equal, there is no shear at the VC coupling, hence minimal drag, or lock up.  If there is a front to rear bias in outer diameter of the tire, then the VC silicone fluid is shearing - and causing some low % of lock up.  Smaller rear diameter causes transfer of power to the front.  smaller front diameter causes drag on the engine at steady cruise speed.  Disclaimer, I was an engineer at a major silicone fluid manufacturer in Germany for 13 years of my career...

 

Hi guys thanks for all of the posts, my manual lists the summer tire sizes as 225/40 ZR 18 front and 295/30 ZR 18 rear, I found this website that was helpful   https://tiresize.com/calculator%C2'> (click on the tire size calculator for rev/mile info) /, Dave is your comment related to the difference in revolutions per mile ? From that web site the difference 804.3 rev/mile front to 808.2 rev/mile rear is that the impact that you are referencing for fluid shear ?  Is that the impact in % difference that xmaciek82x is referencing ?   Should I try to keep those front to back numbers as close to together as possible to eliminate the shear ? And as close to Porsche specs to keep the speedometer and odometer reading correctly ?

 

xmaciek82x does that combination throw off the speedometer, based on that website the rev/mile is listed at 783.6 front & 777 rear ?

 

Thanks

 

Glenn

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The website I pasted in did not get into the above post, is that against forum rules to paste in a link to another website for data reference? the 804.3 & 808.2 rev/miles referenced were for the Porsche summer tires listed in my manual.

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Speedometer?  I've never had a Porsche that was accurate with speed.  I don't worry about it, but others may want to have the speedometer close to actual.. ha ha

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If the tire sizes are exactly equal, there is no shear at the VC coupling, hence minimal drag, or lock up.  If there is a front to rear bias in outer diameter of the tire, then the VC silicone fluid is shearing - and causing some low % of lock up.  Smaller rear diameter causes transfer of power to the front.  smaller front diameter causes drag on the engine at steady cruise speed.  Disclaimer, I was an engineer at a major silicone fluid manufacturer in Germany for 13 years of my career...

I've never known an engineer to be so cavalier about precision, In the spirit of the forum it would have been helpful if you'd just answered the question .....

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If the tire sizes are exactly equal, there is no shear at the VC coupling, hence minimal drag, or lock up.  If there is a front to rear bias in outer diameter of the tire, then the VC silicone fluid is shearing - and causing some low % of lock up.  Smaller rear diameter causes transfer of power to the front.  smaller front diameter causes drag on the engine at steady cruise speed.  Disclaimer, I was an engineer at a major silicone fluid manufacturer in Germany for 13 years of my career...

I've never known an engineer to be so cavalier about precision, In the spirit of the forum it would have been helpful if you'd just answered the question .....

 

Are you serious?  Engineers make decisions every day about precision, and how much is needed.   Unbelievable comment - obviously from a non-engineer.

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Back on topic please - or this thread will be closed.

 

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For the sake of closure and thanks for the advice I wanted to follow up, I intended to return the wheels to 18" but couldn't find what we wanted in a timely manner, a decent looking black 5 spoke. We were looking at the Victor Baden's but couldn't find a full set in 18" or 19" so we opted for staying with 19" rims and chose the Bridgestone Potenza S04 pole position 235/35 19Y tires up front and 305/30 19Y's in the back on Roderick 19" x 11" RW5 's in matte black (about as tight of a fit with this wheel as one could get without spacers). After a month on the car I have to say we are really pleased with the setup, the previous configuration was pretty bone jarring this is the perfect combination of looks, comfort, handling, road noise and performance. Wet weather handling exceeded my expectations.

Thanks again for everyone's input !

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