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oem is ( iirc? ) 1.4% differential btw front X rear. 3% is the *suggested* maximum allowable differences btw F/R so as to not overheat the front diff, with optimal % btw F & R being as close to OEM as is possible. that's why one should run 18's in gt2 sizes and/or remove/disconnect the front diff lol
Moto, I am running 19x8.5 and 19x11. The 225/40/19 and 295/30/19 fits the wheels well and stays with the stock specs as far as overall diameter ratio front vs rear. These widths should be perfectly fine for a street setup (they are stock Turbo widths anyways) Unless we have some definitive proof otherwise...If given the choice I think the safest bet is sticking as close to OEM front VS rear diameter ratios as humanly possible.
Originally Posted by MotoJB
Hi there, thanks for the feedback!
I am currently running 18"rims...8.5 and 11"...with OEM tire sizes. I have an offer for a set of 19's that would be a nice set of street "bling" rims, so I'm simply looking into tire options to see what's viable.
The 19's I'm looking at are 8.5" front with 11" rear.
It would seem your 225/295 suggestion is the most true to MFG recommendations...
Considering this, it's so perplexing why I get so many other suggestions (even from vendors). It's strange to hear so many guys running other combo's and state they have no issues. Does that mean, no issues at the moment...but the diff is definitely being stressed and will eventually wear out due to it?
Let's examine what I have discovered to be a good possible combo, keeping in mind that there is some variance in stock sized tires from the tier 1 brands:
Pirelli P Zero Rossos
225/40: 25.1" tall, 829 revs per mile, tread width 7.6"
295/30: 25.0" tall, 832 revs per mile, tread width 11.0"
Now as it relates to MotoJB's car, he's running the newish Yokohama ADVAN Neova AD08R in stock sizes:
225/40: 25.0" tall, 831 revs per mile, tread width 8.7"
295/30: 25.0" tall, 831 revs per mile, tread width 11.5"
Interestingly enough, the revs are the same front and rear for this particular brand/model.
Even more interesting, check out the next size larger for the fronts:
The Yok 235/40s are 25.3" tall, 820 revs per mile, tread width 9.1".
Would anyone agree that the Yok 235/40 is a better match to their 295/30 as it relates to our cars (that retain the AWD). On paper, this appears to be a viable combo with a slightly taller front with the added benefit of a wider front tire compared to 225s.
Moto, I am running 19x8.5 and 19x11. The 225/40/19 and 295/30/19 fits the wheels well and stays with the stock specs as far as overall diameter ratio front vs rear. These widths should be perfectly fine for a street setup (they are stock Turbo widths anyways) Unless we have some definitive proof otherwise...If given the choice I think the safest bet is sticking as close to OEM front VS rear diameter ratios as humanly possible.
Thanks again for the input gophaster...
BTW, did you have any trouble with the overall taller front/rear fitting (without rubbing)?
Yep, I know that was the previous "perfect" setup for 19's...but too bad there are literally NO good 315/25/19's available in the market.
What tires are you running? Nitto Invo's?
I have Toyo Proxes TR1 which are good but not great. Better than the Invo's in my opinion. I might be upsizing to 26" tires so I can get the Bridgestone Potenza S-04s for my next set though.
I have Toyo Proxes TR1 which are good but not great. Better than the Invo's in my opinion. I might be upsizing to 26" tires so I can get the Bridgestone Potenza S-04s for my next set though.
Thanks man...
Yeah, just about anything is better than Invo's IME. Had them on the car when I bought it and didn't like them at all.
If I get those other rims, I too am considering going to 26" tall 19" tires (26.1" front).
I'm just wondering/concerned if I'll have any issues with rubbing? What have you heard?
I havent heard enough issues with rubbing to be concerned with the tire size. At this point, I will take my chances since the options we have for the current tire size are almost non-existent. The S04s are almost as good as the PS2s and for a fraction of the cost so I feel the gamble is worth it.
I was lucky to score some Michelin tires today, 245/40/18 PSS & 315/30/18 PS2, they fill out the guards very nicely with PSS10's and no rubbing, not that I can tell.
Seems to work well with my AWD.
Ok it seems my front differential/viscous coupling is chewed as I have put winter tires on my Cargraphic 19's and they are a differential profile to the the summer ones, a similar issue to trying to put 997 TT wheels on a 996 TT it seems, you can do it but soon or later you will also knacker the 4WD drivetrain.
after reading these posts just bought a 996 tt and have a 87 dp turbo we bought our wheels and tires thru wheel enhancement the size tires on the 996 are 235 35 19 frt and 305 30 19 rear icalled my tire guy and was informed good luck trting to find a305 30 19 the only one's they carryare nitto and toyo and he said good luck findind either so after talking to my porsche dealer he said they have not had any problems running the 305 30 19 champion porsche sooooo what do you guys think?
after reading these posts just bought a 996 tt and have a 87 dp turbo we bought our wheels and tires thru wheel enhancement the size tires on the 996 are 235 35 19 frt and 305 30 19 rear icalled my tire guy and was informed good luck trting to find a305 30 19 the only one's they carryare nitto and toyo and he said good luck findind either so after talking to my porsche dealer he said they have not had any problems running the 305 30 19 champion porsche sooooo what do you guys think?
I posted this several years ago:
Once you grasp the simplicity and operation of our very basic mechanical AWD system you will realize that it is absolutely imperative that the rear wheels turn JUST A TINY BIT faster than the front wheels to have any kind of constant torque transfer from rear to front in a situation of a car being driven in a straight line at a constant speed. Either that or you need to have a different front and final drive ratios but in our case of the 996tt they are both the same at 3.44 according to Porsche. With that said, diameter "delta" between the front and rear will determine the amount of torque transferred from rear to front (the smaller the diameter of the rears relative to the fronts, the more torque will be transferred to the front). Obviously you can't go with too great a difference or you will overheat the silicone based fluid in the viscous coupling due to constant slippage. Most people who have run SLIGHTLY larger tires in the rear are fine with regards to damaging the viscous coupler if they stay with in the variance mandated by Porsche and they probably can't tell or "feel" the small amount of negative torque (driveline drag) which is created by the front wheels. You can think of it as your front brakes dragging just a tiny bit. The larger the rear tires get relative to the fronts the more negative torque (drag) you are transferring from the front to rear. To put in another way, if both your front and rear wheels were the exact same diameter than you would get zero torque to the front unless the rear wheels began spinning at which point a certain amount of power (up to 30%) would get transferred to the front wheels.
To look at this from the point of the viscous coupler, when the front wheels are smaller then the rears (as in a 235/35/19 and 305/30/19 combo) they will obviously spin faster then the rears. When the fronts spin faster then the rears then they drive their respective vanes within the viscous coupler faster than the vanes connected the the cardan shaft / rear drive. In essence, the front wheels are transferring torque to the rear through the viscous coupler. This happens until the rears start rotating faster due to slippage for example. An easy analogy would be this: You have a pipe with a motor on the front of the pipe turning a propeller midway in the pipe facing another propeller connected to a motor at the back of the pipe. If the front motor spins the propeller faster then it will push whatever "medium" is inside past the slower turning propeller in the rear. The front wheel drive portion of our AWD system is incredibly basic when you really grasp the simplicity of it's operation. Hope this helps....
The only limitation that Porsche puts on the AWD system viscous coupler is the 5mph difference between the rotation of the front and rear wheels which works out to 21 revs per mile difference of the front to rear axle at 180mph. This limitation most likely has to do with the overheating of the temperature sensitive silicone fluid due to rotational differences of the input/output vanes in the viscous coupler.
Here is a good read that explains the AWD system on our cars on page 257-259:
Once you grasp the simplicity and operation of our very basic mechanical AWD system you will realize that it is absolutely imperative that the rear wheels turn JUST A TINY BIT faster than the front wheels to have any kind of constant torque transfer from rear to front in a situation of a car being driven in a straight line at a constant speed. Either that or you need to have a different front and final drive ratios but in our case of the 996tt they are both the same at 3.44 according to Porsche. With that said, diameter "delta" between the front and rear will determine the amount of torque transferred from rear to front (the smaller the diameter of the rears relative to the fronts, the more torque will be transferred to the front). Obviously you can't go with too great a difference or you will overheat the silicone based fluid in the viscous coupling due to constant slippage. Most people who have run SLIGHTLY larger tires in the rear are fine with regards to damaging the viscous coupler if they stay with in the variance mandated by Porsche and they probably can't tell or "feel" the small amount of negative torque (driveline drag) which is created by the front wheels. You can think of it as your front brakes dragging just a tiny bit. The larger the rear tires get relative to the fronts the more negative torque (drag) you are transferring from the front to rear. To put in another way, if both your front and rear wheels were the exact same diameter than you would get zero torque to the front unless the rear wheels began spinning at which point a certain amount of power (up to 30%) would get transferred to the front wheels.
To look at this from the point of the viscous coupler, when the front wheels are smaller then the rears (as in a 235/35/19 and 305/30/19 combo) they will obviously spin faster then the rears. When the fronts spin faster then the rears then they drive their respective vanes within the viscous coupler faster than the vanes connected the the cardan shaft / rear drive. In essence, the front wheels are transferring torque to the rear through the viscous coupler. This happens until the rears start rotating faster due to slippage for example. An easy analogy would be this: You have a pipe with a motor on the front of the pipe turning a propeller midway in the pipe facing another propeller connected to a motor at the back of the pipe. If the front motor spins the propeller faster then it will push whatever "medium" is inside past the slower turning propeller in the rear. The front wheel drive portion of our AWD system is incredibly basic when you really grasp the simplicity of it's operation. Hope this helps....
The only limitation that Porsche puts on the AWD system viscous coupler is the 5mph difference between the rotation of the front and rear wheels which works out to 21 revs per mile difference of the front to rear axle at 180mph. This limitation most likely has to do with the overheating of the temperature sensitive silicone fluid due to rotational differences of the input/output vanes in the viscous coupler.
Here is a good read that explains the AWD system on our cars on page 257-259:
(the smaller the diameter of the rears relative to the fronts, the more torque will be transferred to the front). Obviously you can't go with too great a difference or you will overheat the silicone based fluid in the viscous coupling due to constant slippage.
The only limitation that Porsche puts on the AWD system viscous coupler is the 5mph difference between the rotation of the front and rear wheels which works out to 21 revs per mile difference of the front to rear axle at 180mph.
So same dia. front to rear means zero torque transfer until the rear wheels spin.
What about this delta with the fronts being .3" taller than the rear. Based on what you've said, this appears to be a safe combo And I'm very confident that I will never see 180mph in my car.
235/40: 25.3" tall, 820 revs per mile, tread width 9.1"
295/30: 25.0" tall, 831 revs per mile, tread width 11.5"
So same dia. front to rear means zero torque transfer until the rear wheels spin.
What about this delta with the fronts being .3" taller than the rear. Based on what you've said, this appears to be a safe combo And I'm very confident that I will never see 180mph in my car.
235/40: 25.3" tall, 820 revs per mile, tread width 9.1"
295/30: 25.0" tall, 831 revs per mile, tread width 11.5"
Thanks pwdrhound.
Yes, that is perfectly fine. My recommendation for AWD no frills bolt of set up without extensive mods is 245/40/18 and 315/30/18
07-07-2016, 11:24 AM
Had them on the car when I bought it and didn't like them at all.
And I'm very confident that I will never see 180mph in my car.