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Old 05-22-2019, 04:55 AM   #21
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On a side note, check out this video. You may have to change your signature.

https://www.youtube.com/watch?v=POSKNO7z5Sw
Neat vid - thanks. But change my signature? Not. A. Chance.

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Old 05-22-2019, 05:01 AM   #22
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Yes, mount some fresh tires ASAP because 12 yr old tires suck! Ideal sizes for those wheels on a 986 are 235/40/18 and 265/35/18.

Pretty fun discussion on tire diameter differences though. "Twitchiness"??

All 987, 997, 981, 991 run 1" smaller diameter in front than the rear. It adds a little rake to the car so there is more weight on the front tires. This helps to generate excellent car balance in the corners. The idea that all these cars are "twitchy" because of the different diameter has never been discovered until now.

If your car feels twitchy, it needs an alignment.
Well you learned something today again then. On a alignment for street car, the tire rotation makes a big difference when you decide to go for a spirited drive. No my car does not need an alignment as I can fell the twitchyness changing the front tire to a smaller faster rotating speed.

People that race, align their cars much different then street cars. Alignments good for race cars are not good for street cars, if you want to have proper tire wear.

"All 987, 997, 981, 991 run 1" smaller diameter in front than the rear."? You are completely wrong in this statement.
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Old 05-22-2019, 10:56 AM   #23
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Originally Posted by wrs986 View Post
This car was so well cared for but the previous owner was not a “car guy” and I figured the tires would need to be replaced ASAP.
How far off will my speedometer be if I go from 265/40/18 to 265/35/18?
Right now it’s within 1 mph of my GPS. Is the speedometer reading taken from the rear or front tires? I’ll run down to tire rack next week and replace the Potenzas. What was oem for this car?
The speedometer reading comes from averaging the abs speed sensors from all 4 wheels I believe. I read it on the internet so it must be true! If you use the stock 18 inch tire sizes your speedometer will read about 2-3 mph higher than what a mobile phone GPS speedometer app is showing. There is an EU regulation or German law that fines auto manufacturers for all cars they sell whose speedometers read lower than actual speed. We've owned 1 Audi, 3 BMW's and 1 Porsche and that's always been our speedometer experience with German cars. And then there is a US law that requires all auto manufacturers to have accurate odometers so the warranties don't run out sooner than they should. So with the right sized tires your speedometer is going to read a little higher than actual speed and your odometer is going to be accurate.
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Old 05-22-2019, 11:46 AM   #24
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Well you learned something today again then. On a alignment for street car, the tire rotation makes a big difference when you decide to go for a spirited drive. No my car does not need an alignment as I can fell the twitchyness changing the front tire to a smaller faster rotating speed.

People that race, align their cars much different then street cars. Alignments good for race cars are not good for street cars, if you want to have proper tire wear.

"All 987, 997, 981, 991 run 1" smaller diameter in front than the rear."? You are completely wrong in this statement.
Negative ghostrider, stock 981 Boxsters typically run 26" diameter in front and 27" diameter in rear. I ride right seat in a LOT of 987/997/981/991 cars at the race track as an instructor. No twitch, no unusual tire wear as long as the alignment was set up for the type of driving they do.

I drove my street 986 for 10 years and 100 track days experimenting with a lot of different tire sizes, no twitch unless I had too much rear sway bar.

Here are stock sizes for a 2013 Boxster. Note the difference F/R in diameter and revs per mi. Different sizes, no twitch.
https://www.tirerack.com/tires/tires.jsp?tireMake=Bridgestone&tireModel=Potenza+RE-71R&frontTire=345WR8RE71R&rearTire=645WR8RE71RXL&vehicleSearch=true&fromCompare1=yes&autoMake=Porsche&autoYear=2013&autoModel=Boxster&autoModClar=


My 2009 Cayman runs 25"F, 26"R as stock sizes.
https://www.tirerack.com/tires/tires.jsp?tireMake=Michelin&tireModel=Pilot+Sport+4S&frontTire=34YR8PS4SXL&rearTire=64YR8PS4SXL&vehicleSearch=true&fromCompare1=yes&autoMake=Porsche&autoYear=2009&autoModel=Cayman&autoModClar=

Here are stock sizes for a 987.2 Carrera. 25"F, 26" R
https://www.tirerack.com/tires/tires.jsp?tireMake=Michelin&tireModel=Pilot+Sport+PS2&frontTire=34YR8SPTPS2XLN4&rearTire=64YR8SPORTPS2XLN4&vehicleSearch=true&fromCompare1=yes&autoMake=Porsche&autoYear=2009&autoModel=911%20Carrera%20Coupe&autoModClar=

I think somebody has been yanking your chain a bit KRAM.
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Old 05-22-2019, 12:27 PM   #25
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Negative ghostrider, stock 981 Boxsters typically run 26" diameter in front and 27" diameter in rear. I ride right seat in a LOT of 987/997/981/991 cars at the race track as an instructor. No twitch, no unusual tire wear as long as the alignment was set up for the type of driving they do.

I drove my street 986 for 10 years and 100 track days experimenting with a lot of different tire sizes, no twitch unless I had too much rear sway bar.

Here are stock sizes for a 2013 Boxster. Note the difference F/R in diameter and revs per mi. Different sizes, no twitch.
https://www.tirerack.com/tires/tires.jsp?tireMake=Bridgestone&tireModel=Potenza+RE-71R&frontTire=345WR8RE71R&rearTire=645WR8RE71RXL&vehicleSearch=true&fromCompare1=yes&autoMake=Porsche&autoYear=2013&autoModel=Boxster&autoModClar=


My 2009 Cayman runs 25"F, 26"R as stock sizes.
https://www.tirerack.com/tires/tires.jsp?tireMake=Michelin&tireModel=Pilot+Sport+4S&frontTire=34YR8PS4SXL&rearTire=64YR8PS4SXL&vehicleSearch=true&fromCompare1=yes&autoMake=Porsche&autoYear=2009&autoModel=Cayman&autoModClar=

Here are stock sizes for a 987.2 Carrera. 25"F, 26" R
https://www.tirerack.com/tires/tires.jsp?tireMake=Michelin&tireModel=Pilot+Sport+PS2&frontTire=34YR8SPTPS2XLN4&rearTire=64YR8SPORTPS2XLN4&vehicleSearch=true&fromCompare1=yes&autoMake=Porsche&autoYear=2009&autoModel=911%20Carrera%20Coupe&autoModClar=

I think somebody has been yanking your chain a bit KRAM.
Not negative. Just because a number goes from 26 to 27 does not make it a full 1", nor does your statement that ALL 987, 997, 981, 991 run 1" smaller diameter in front than the rear hold true.

I can tell the difference, you can't, no big deal.

Only person yanking chains is you. My Boxster is my DD and it's my only vehicle, it's been that way since 2014. I notice everything on my car when something changes. I personally do not like faster rotating front tires, it throws off the balance to me.

I'll will always recommend getting your tires to rotate as close as possible to the same speed. Nothing wrong or illogical in my recommendation and you're not going to change that.
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Old 05-22-2019, 02:52 PM   #26
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Carry on KRAM...
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Old 05-22-2019, 02:52 PM   #27
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My Boxster is my DD and it's my only vehicle, it's been that way since 2014. I notice everything on my car when something changes. I personally do not like faster rotating front tires, it throws off the balance to me.
You remind me of my 1st wife, who swore she could smell cats, when there were no cats.
I think much of what you THINK you notice is what I call "confirmation bias", or a simple psychosomatic response. You expect to feel a difference (you even WANT to feel a difference) and so you feel a difference.
Blindfold you and don't tell you what tires are on (or which quality-brand spark plugs are in the motor) and you won't feel the difference any longer.



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Old 05-22-2019, 02:56 PM   #28
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You remind me of my 1st wife, who swore she could smell cats, when there were no cats.
I think much of what you THINK you notice is what I call "confirmation bias", or a simple psychosomatic response. You expect to feel a difference (you even WANT to feel a difference) and so you feel a difference.
Blindfold you and don't tell you what tires are on (or which quality-brand spark plugs are in the motor) and you won't feel the difference any longer.



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Oh ya this is spark plug horsepower guy. nuff said.
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Old 05-22-2019, 03:11 PM   #29
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You remind me of my 1st wife, who swore she could smell cats, when there were no cats.
I think much of what you THINK you notice is what I call "confirmation bias", or a simple psychosomatic response. You expect to feel a difference (you even WANT to feel a difference) and so you feel a difference.
Blindfold you and don't tell you what tires are on (or which quality-brand spark plugs are in the motor) and you won't feel the difference any longer.



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Change it on my car and I will notice the change without being told.

Why are you even commenting? Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car? Or are you just being a chain jerkier like Topless?
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Old 05-22-2019, 03:16 PM   #30
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Change it on my car and I will notice the change without being told.



Why are you even commenting? Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car? Or are you just being a chain jerkier like Topless?
Because people use these forums as a knowledge base. If ridiculousness is left unchallenged, it gets defacto credibility.

That's why I'm commenting.

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Old 05-22-2019, 03:19 PM   #31
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Blindfold you and don't tell you what tires are on (or which quality-brand spark plugs are in the motor) and you won't feel the difference any longer.

Driving blindfolded. Now there's an idea! Show us how it's done, Maytag!
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Old 05-22-2019, 03:23 PM   #32
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Because people use these forums as a knowledge base. If ridiculousness is left unchallenged, it gets defacto credibility.

That's why I'm commenting.

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You said nothing to discredit my recommendation. You've put nothing in check. You're just showing you want to argue just like Topless with zero bases to argue from.
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Old 05-22-2019, 03:23 PM   #33
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Driving blindfolded. Now there's an idea! Show us how it's done, Maytag!
https://beta.washingtonpost.com/technology/2019/01/12/driving-blindfolded-bird-box-challenge-just-dont-officials-say/?outputType=amp


Can't make this stuff up, Haha

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Old 05-22-2019, 03:27 PM   #34
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C'mon KRAM, put up or shut up.

Lets see hard data for all the 987/997/981/991 cars that came from the Porsche factory with identical or nearly identical (less than .3" difference) F/R tire diameters. Google is your friend and there are a few, and I know which ones. The numbers are out there for all to see. If you are going to make an argument AT LEAST be able to support that argument with hard facts instead of butt dyno mumbo jumbo.

You have 30 minutes to demonstrate your awesomeness to all who visit here.

Go!
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Old 05-22-2019, 03:33 PM   #35
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C'mon KRAM, put up or shut up.

Lets see hard data for all the 987/997/981/991 cars that came from the Porsche factory with identical or nearly identical (less than .3" difference) F/R tire diameters. Google is your friend and there are a few... very few and I know which ones. The numbers are out there for all to see. If you are going to make an argument AT LEAST be able to support that argument with hard facts instead of butt dyno mumbo jumbo.

You have 30 minutes to demonstrate your awesomeness to all who visit here.

Go!
I don't have 30 min to do anything.

You made the false claim. Two of you examples were not 1" different.

Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car?
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Old 05-22-2019, 03:50 PM   #36
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I don't have 30 min to do anything.

You made the false claim. Two of you examples were not 1" different.

Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car?
Yes, glad you asked. There is a very valid reason. On 987,981, and most 997 cars the PSM and ABS are precisely calibrated to stock tire stagger in diameter (typically 1" differential). Put the same diameter tire on these cars and the PSM overreacts in corners applying the rear brakes and cutting the throttle. It can also confuse ABS creating an "ice pedal" condition under heavy braking causing a crash.

Getting the factory tire diameter stagger is very important in Porsche cars after 2005 so that PSM and ABS function properly. Mounting the same diameter tire on all cars without consideration of factory designed stagger is foolish and dangerous. The 986 has a much simpler PSM and ABS that do not require precise matching of tire diameter. The OP can happily run the tires he is on with little or no effect. If he does change the rear to a 265/35/18 he will notice a bit better acceleration and his speedo will again be off by 4-6mph.
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Old 05-22-2019, 04:23 PM   #37
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OMG! LOL I hope this doesn't spread like the Tide Pod challenge! Good lord, were we that stupid then?
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Old 05-22-2019, 04:46 PM   #38
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Do you have a valid reason not to try and match the front and rear tire rotation speed best as possible on a street car?
Hey man, don't start throwing out squirrels here..... you're changing the direction to deflect....
This is about whether or not you can FEEL it..... hahaha.

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Old 05-22-2019, 04:58 PM   #39
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Yes, glad you asked. There is a very valid reason. On 987,981, and most 997 cars the PSM and ABS are precisely calibrated to stock tire stagger in diameter (typically 1" differential). Put the same diameter tire on these cars and the PSM overreacts in corners applying the rear brakes and cutting the throttle. It can also confuse ABS creating an "ice pedal" condition under heavy braking causing a crash.

Getting the factory tire diameter stagger is very important in Porsche cars after 2005 so that PSM and ABS function properly. Mounting the same diameter tire on all cars without consideration of factory designed stagger is foolish and dangerous.
I said street cars, not race cars were "ice pedal" is a common issue and I have yet to read any racers blaming it on the tire circumference size. I've read racers having a too aggressive master cylinder setups along with running tires that are basically slicks, to pads and rotors are not stopping the rotation of your wheels enough to reach lockup.

PCA even made a 2019 the proposed rule change. "Every ABS whose PWIS programming includes an option for a PCCB flash may use that flash. Doing this seems to help with "ice" pedal and is inexpensive."

I thought you were a "PCA-GPX Chief Driving Instructor"? You do give breaking lessons to help keep racers out of this issue as much as possible, right?

On a street car "ice pedal" can happen with just hitting a bump in the road at the wrong time and confusing the ABS. I have never had a "ice pedal" issue in my car, ever, and I'm running smaller wheels and tires then my car came with from factory. Not by choice as someone swiped the Carrera Light 18" wheels off the car, probably one of the dealerships the car was at as I'm the 4th owner of the car.

People change the wheels and tire size on their trucks, SUV and cars all the time. Going from 18 up to 22" on trucks and SUV, some people will even do it on cars lol and never ever do you hear about people having "ice pedal" issues. Heck you can walk onto a dealership lot and they will sell you a customized Truck/SUV with way bigger tires then stock and the dealership won't break a sweat over it because it's just not a street issue. If it was, no dealership in the USA would sell you a vehicle setup like that as they would get sued for putting the people in danger and them crashing the vehicle. Way too much fear mongering with that post.

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Hey man, don't start throwing out squirrels here..... you're changing the direction to deflect....
This is about whether or not you can FEEL it..... hahaha.

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I can feel the difference. The car is more balanced through the corners when the front and rear tires are as close as can be to rotational speed. There is a lot less steering input needed. I'm not fighting the car around the corner, the balance is great. Maybe you should try it? Oh never mind on that, I see you have had a track alignment done to your car.
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Old 05-22-2019, 05:55 PM   #40
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