Tires Sizes
Registered User
Joined: Sep 2010
Posts: 23
From: The city
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not trying to hijack this tread but,
i have a gt2 and am getting some tech1 wheels for it, and want to add a little more rubber, from the 325 to 345 in rear and 235 to 245 in front.
is this all kosher? will i experience any rubbing? stock height car. can/should i go 255 in front? will that distract from steering response/feel? or?
thank you.
i have a gt2 and am getting some tech1 wheels for it, and want to add a little more rubber, from the 325 to 345 in rear and 235 to 245 in front.
is this all kosher? will i experience any rubbing? stock height car. can/should i go 255 in front? will that distract from steering response/feel? or?
thank you.
Registered User
Joined: Dec 2006
Posts: 3,116
From: Palos Verdes
Rep Power: 256
You are spot-on, of course. The onboard computers on the Turbo constantly monitor the speed RELATIONSHIP between front and rear.
If the front turns slower, and the rear turns slower to a similar percentage, no problem.
However, if the rears turn too fast in relation to the front, the CPU thinks there is a loss of traction and splits the torque to the front wheels. That's fine, occasionally. But if there is constant torque split, the diff will overheat, the CPU will severely detune the engine (limp home mode) and all kinds of warning and overheat lights come on.
Therefore, the important thing in tire choice is to maintain the correct "static loaded radius" relationship between front and rear.
Then the computers stay happy and the smilie light on the dash comes on.
Regarding the compensation capability of the computers, or flexibility in the tire speed relationship, the car must be able to tolerate the approximate 2% wear factor of the tires from new to old. The front and rear tires do not wear at the same rate. I am told by many experts that as long as the difference in revolutions per mile doesn't exceed 2.5%, you are safe.
Of course, you could ask Porsche, but you might as well ask the tire.
If the front turns slower, and the rear turns slower to a similar percentage, no problem.
However, if the rears turn too fast in relation to the front, the CPU thinks there is a loss of traction and splits the torque to the front wheels. That's fine, occasionally. But if there is constant torque split, the diff will overheat, the CPU will severely detune the engine (limp home mode) and all kinds of warning and overheat lights come on.
Therefore, the important thing in tire choice is to maintain the correct "static loaded radius" relationship between front and rear.
Then the computers stay happy and the smilie light on the dash comes on.
Regarding the compensation capability of the computers, or flexibility in the tire speed relationship, the car must be able to tolerate the approximate 2% wear factor of the tires from new to old. The front and rear tires do not wear at the same rate. I am told by many experts that as long as the difference in revolutions per mile doesn't exceed 2.5%, you are safe.
Of course, you could ask Porsche, but you might as well ask the tire.
1. Increasing tire width increases traction. But... what is the *disadvantage* of increasing tire width? Surely there must be something. Maybe increasing front tire width will decrease steering response and "lightness," for example?
2. My front rim is 8.5: is this wide enough for a 245 tire? Asking the question in another way:
A. What is the max tire width does the rim width of 8.5 support, for the front?
B. What is the max tire width does the rim width of 11 support, for the rear?
Thanks again.
Registered User
Joined: Mar 2008
Posts: 642
From: Kansas
Rep Power: 79
1. Increasing tire width does increase grip. But total overall traction is controlled by a variety of factors like engine power, tire compound, temperature of the tire and the pavement, computer controls, gear ratio, etc.
There are disadvantages of course. Wider tires are usually heavier and that's bad (I've posted a lot about this). Wider tires definitely affect the handling and steering response, sometimes for the better, sometimes not. Wider tires have a greater tendency to hydroplane and also have less traction on snowy surfaces. Wider tires will increase rolling resistance, so sometimes you trade low-end traction and cornering grip for top speed.
Wider tires are usually more expensive. But this can make your wallet lighter so maybe that's a trade-off as well
2. An 8.5 wide front wheel is certainly adequate for a 245 tire. The tire companies always publish the rim width range for each tire size they make.
However, there is always an OPTIMAL width. If the wheel is narrow, the tire will "crown" and lose grip. If the wheel is wide, the sidewall will stretch and you will lose ride comfort but increase steering response. All compromises.
There is usually about 1.5 to 2 inch range in recommended rim width for each tire size. However, the lower the profile of the tire, the tighter the range on the rim width because there is less sidewall available.
If you visit tirerack.com, they list all the tire sizes and recommended rim widths for all the tires they sell.
An 11 inch wheel can accept tire widths from 275 to 305, depending upon the tire company and aspect ratio desired.
Thanks for the kind words.
Mike
There are disadvantages of course. Wider tires are usually heavier and that's bad (I've posted a lot about this). Wider tires definitely affect the handling and steering response, sometimes for the better, sometimes not. Wider tires have a greater tendency to hydroplane and also have less traction on snowy surfaces. Wider tires will increase rolling resistance, so sometimes you trade low-end traction and cornering grip for top speed.
Wider tires are usually more expensive. But this can make your wallet lighter so maybe that's a trade-off as well
2. An 8.5 wide front wheel is certainly adequate for a 245 tire. The tire companies always publish the rim width range for each tire size they make.
However, there is always an OPTIMAL width. If the wheel is narrow, the tire will "crown" and lose grip. If the wheel is wide, the sidewall will stretch and you will lose ride comfort but increase steering response. All compromises.
There is usually about 1.5 to 2 inch range in recommended rim width for each tire size. However, the lower the profile of the tire, the tighter the range on the rim width because there is less sidewall available.
If you visit tirerack.com, they list all the tire sizes and recommended rim widths for all the tires they sell.
An 11 inch wheel can accept tire widths from 275 to 305, depending upon the tire company and aspect ratio desired.
Thanks for the kind words.
Mike
Mike, thanks for the excellent explanation; I always seem to learn something when I trade posts with you. More questions please:
1. Increasing tire width increases traction. But... what is the *disadvantage* of increasing tire width? Surely there must be something. Maybe increasing front tire width will decrease steering response and "lightness," for example?
2. My front rim is 8.5: is this wide enough for a 245 tire? Asking the question in another way:
A. What is the max tire width does the rim width of 8.5 support, for the front?
B. What is the max tire width does the rim width of 11 support, for the rear?
Thanks again.
1. Increasing tire width increases traction. But... what is the *disadvantage* of increasing tire width? Surely there must be something. Maybe increasing front tire width will decrease steering response and "lightness," for example?
2. My front rim is 8.5: is this wide enough for a 245 tire? Asking the question in another way:
A. What is the max tire width does the rim width of 8.5 support, for the front?
B. What is the max tire width does the rim width of 11 support, for the rear?
Thanks again.
Registered User
Joined: Mar 2008
Posts: 642
From: Kansas
Rep Power: 79
Dan,
The important thing is to manage the relative speed of the front and rear tires. This keeps the software and traction control systems happy.
The 345/30R19 Viper tire is a bit taller than the 325 Cups that came on your car. So you need a front tire that is a bit taller as well. The 255 front fills this bill but is very tight under the front fenders.
PM me or email me and I will do my best to help you.
Mike
The important thing is to manage the relative speed of the front and rear tires. This keeps the software and traction control systems happy.
The 345/30R19 Viper tire is a bit taller than the 325 Cups that came on your car. So you need a front tire that is a bit taller as well. The 255 front fills this bill but is very tight under the front fenders.
PM me or email me and I will do my best to help you.
Mike
not trying to hijack this tread but,
i have a gt2 and am getting some tech1 wheels for it, and want to add a little more rubber, from the 325 to 345 in rear and 235 to 245 in front.
is this all kosher? will i experience any rubbing? stock height car. can/should i go 255 in front? will that distract from steering response/feel? or?
thank you.
i have a gt2 and am getting some tech1 wheels for it, and want to add a little more rubber, from the 325 to 345 in rear and 235 to 245 in front.
is this all kosher? will i experience any rubbing? stock height car. can/should i go 255 in front? will that distract from steering response/feel? or?
thank you.
Registered User
Joined: Aug 2009
Posts: 946
From: Kansas
Rep Power: 83
Dan,
The important thing is to manage the relative speed of the front and rear tires. This keeps the software and traction control systems happy.
The 345/30R19 Viper tire is a bit taller than the 325 Cups that came on your car. So you need a front tire that is a bit taller as well. The 255 front fills this bill but is very tight under the front fenders.
PM me or email me and I will do my best to help you.
Mike
The important thing is to manage the relative speed of the front and rear tires. This keeps the software and traction control systems happy.
The 345/30R19 Viper tire is a bit taller than the 325 Cups that came on your car. So you need a front tire that is a bit taller as well. The 255 front fills this bill but is very tight under the front fenders.
PM me or email me and I will do my best to help you.
Mike
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