Tire Size & AWD
07-26-2015, 06:49 PM
#1
Tire Size & AWD
I have a questions for the tire experts. Please excuse my new-guy ignorance:
I have a 991 C4 and I've recently purchased a set of wheels for track/winter use from a fellow Rennlist member. The tires that came with the wheels are slightly different from the stock 19" specs:
Yokohama AD08-R
Front: 245/35-19
Rear: 305/30-19
Using an online calculator, the % diameter difference is 1.5%, circumference 1.7%
The stock 20" tire specs on my car at the moment are the same as these tires (245/35-20, 305/30-20) however I've noticed that on the Porsche recommended 19" wheel sets, the tires recommended are slightly different:
Front: 235/40-19
Rear: 295/35-19
The same calculator has the % difference of diameter is 2.6%, circumference 2.8%
I've been reading a bit about % difference in diameter/circumference and I was wondering if the difference between the front and the rear axel is more important than the deviation from the stock Porsche tire spec. Will the ECU have difficulties with the non-spec tires? Will my AWD system be fine?
I don't want to mess with warranty issues or have any car troubles. Many thanks in advance for your advice.
I have a 991 C4 and I've recently purchased a set of wheels for track/winter use from a fellow Rennlist member. The tires that came with the wheels are slightly different from the stock 19" specs:
Yokohama AD08-R
Front: 245/35-19
Rear: 305/30-19
Using an online calculator, the % diameter difference is 1.5%, circumference 1.7%
The stock 20" tire specs on my car at the moment are the same as these tires (245/35-20, 305/30-20) however I've noticed that on the Porsche recommended 19" wheel sets, the tires recommended are slightly different:
Front: 235/40-19
Rear: 295/35-19
The same calculator has the % difference of diameter is 2.6%, circumference 2.8%
I've been reading a bit about % difference in diameter/circumference and I was wondering if the difference between the front and the rear axel is more important than the deviation from the stock Porsche tire spec. Will the ECU have difficulties with the non-spec tires? Will my AWD system be fine?
I don't want to mess with warranty issues or have any car troubles. Many thanks in advance for your advice.
08-04-2015, 10:16 PM
#3
The way the OP states their question makes me want to clarify.
What matters with the AWD systems is that you maintain the ratio of the size of the front to rear tire. So if Porsche says to use:
Front: 235/40-19
Rear: 295/35-19
You go to one of the many tire websites and look up the specs for these tires using a brand of tire that Porsche uses, lets say Michelin. Those specs might be (I am making these up)
for the the 235 a diameter of 25.5 inches and 817 revolutions per mile
for the 295 a diameter of 26.3 inches and 792 revs.
The ratio of front / back is then 25.5/26.3 = 0.970
The ratio is what matters, for any given size front you can find to use (245/35/19) you should find a back that results in a ratio as close to the stock ratio as possible.
I have read tons of posting on this and the general opinion is that if you stay within 1% of the stock ratio you are OK. Many but not all posts say you can go up to 2-3% without problem but that over 3% is asking for failure. I am NOT saying I have any experience with this, only providing a summary. I never saw a posting that could site specifically a Porsche authority on this.
It is a good idea to verify the size specs for the specific brands and models of tires you are considering, not all 245/35/19 will be the same.
For what its worth, for my 997 C4S I just purchased a set of 18 inch track tires and am deviating slightly from the stock ratio and wont be worrying about it.
What matters with the AWD systems is that you maintain the ratio of the size of the front to rear tire. So if Porsche says to use:
Front: 235/40-19
Rear: 295/35-19
You go to one of the many tire websites and look up the specs for these tires using a brand of tire that Porsche uses, lets say Michelin. Those specs might be (I am making these up)
for the the 235 a diameter of 25.5 inches and 817 revolutions per mile
for the 295 a diameter of 26.3 inches and 792 revs.
The ratio of front / back is then 25.5/26.3 = 0.970
The ratio is what matters, for any given size front you can find to use (245/35/19) you should find a back that results in a ratio as close to the stock ratio as possible.
I have read tons of posting on this and the general opinion is that if you stay within 1% of the stock ratio you are OK. Many but not all posts say you can go up to 2-3% without problem but that over 3% is asking for failure. I am NOT saying I have any experience with this, only providing a summary. I never saw a posting that could site specifically a Porsche authority on this.
It is a good idea to verify the size specs for the specific brands and models of tires you are considering, not all 245/35/19 will be the same.
For what its worth, for my 997 C4S I just purchased a set of 18 inch track tires and am deviating slightly from the stock ratio and wont be worrying about it.
08-05-2015, 09:58 PM
#4
The way the OP states their question makes me want to clarify.
What matters with the AWD systems is that you maintain the ratio of the size of the front to rear tire. So if Porsche says to use:
Front: 235/40-19
Rear: 295/35-19
You go to one of the many tire websites and look up the specs for these tires using a brand of tire that Porsche uses, lets say Michelin. Those specs might be (I am making these up)
for the the 235 a diameter of 25.5 inches and 817 revolutions per mile
for the 295 a diameter of 26.3 inches and 792 revs.
The ratio of front / back is then 25.5/26.3 = 0.970
The ratio is what matters, for any given size front you can find to use (245/35/19) you should find a back that results in a ratio as close to the stock ratio as possible.
I have read tons of posting on this and the general opinion is that if you stay within 1% of the stock ratio you are OK. Many but not all posts say you can go up to 2-3% without problem but that over 3% is asking for failure. I am NOT saying I have any experience with this, only providing a summary. I never saw a posting that could site specifically a Porsche authority on this.
It is a good idea to verify the size specs for the specific brands and models of tires you are considering, not all 245/35/19 will be the same.
For what its worth, for my 997 C4S I just purchased a set of 18 inch track tires and am deviating slightly from the stock ratio and wont be worrying about it.
What matters with the AWD systems is that you maintain the ratio of the size of the front to rear tire. So if Porsche says to use:
Front: 235/40-19
Rear: 295/35-19
You go to one of the many tire websites and look up the specs for these tires using a brand of tire that Porsche uses, lets say Michelin. Those specs might be (I am making these up)
for the the 235 a diameter of 25.5 inches and 817 revolutions per mile
for the 295 a diameter of 26.3 inches and 792 revs.
The ratio of front / back is then 25.5/26.3 = 0.970
The ratio is what matters, for any given size front you can find to use (245/35/19) you should find a back that results in a ratio as close to the stock ratio as possible.
I have read tons of posting on this and the general opinion is that if you stay within 1% of the stock ratio you are OK. Many but not all posts say you can go up to 2-3% without problem but that over 3% is asking for failure. I am NOT saying I have any experience with this, only providing a summary. I never saw a posting that could site specifically a Porsche authority on this.
It is a good idea to verify the size specs for the specific brands and models of tires you are considering, not all 245/35/19 will be the same.
For what its worth, for my 997 C4S I just purchased a set of 18 inch track tires and am deviating slightly from the stock ratio and wont be worrying about it.
I'm assuming that AWD system only really "sees" the relative difference between the front and rear axels, so a 19" wheel set up with 245/35, 305/30 would be interpreted exactly the same as a 20" wheel set with the exact same tire specs.
Do I have that correct?
08-06-2015, 12:07 AM
#5
Thanks for the reply.
I'm assuming that AWD system only really "sees" the relative difference between the front and rear axels, so a 19" wheel set up with 245/35, 305/30 would be interpreted exactly the same as a 20" wheel set with the exact same tire specs.
Do I have that correct?
I'm assuming that AWD system only really "sees" the relative difference between the front and rear axels, so a 19" wheel set up with 245/35, 305/30 would be interpreted exactly the same as a 20" wheel set with the exact same tire specs.
Do I have that correct?
I cannot confirm or deny your statement about 19" Vs 20" wheels with the same specs. For example, according to tirerack a Michelin PSS
245/35/19 is 25.8" diameter
245/35/20 is 26.8" diameter
305/30/19 is 26.3" diameter
305/30/19 is 27.2" diameter
Front / Rear ratios:
19s = 0.981
20s = 0.985
So even though your are using the same brand and same tire specs (specs meaning aspect ratio, width, etc) you can see from the information on diameters that the ratio of tire diameter front to rear is actually slightly different in this example. In this case they are close enough that by all non official information I have seen they are fine. I don't have specific knowledge to generalize about different tire brands, etc so would always just run the numbers to be safe.
08-06-2015, 12:47 AM
#6
One more point. In the case of using the same tire specs but 19" vs 20" you would be changing the height of the car, handling to some degree and there could be serious clearance issues.
If you look in your owners manual you would see that when Porsche indicates different wheel sizes are allowed they do change the tire specs to keep overall dimension of the tire similar.
If you look in your owners manual you would see that when Porsche indicates different wheel sizes are allowed they do change the tire specs to keep overall dimension of the tire similar.