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GT4 Rear Tire Size

13K views 13 replies 9 participants last post by  Westcoaster 
#1 ·
Has anyone tried a 305x30x20 on the rear of their GT4? I have a chance to pick up a slightly used set of Cup 2 tires off of a GT3. The fronts of course are the same as the GT4 but the rears are 305x30. Has anyone tried this size on the rear? I only plan to use them on the street. I have a set of 19" for the track.
 
#2 ·
A fellow I know tracking a GT4 said he heard a 305 19" will fit, but I do not know for sure if anyone has done it. I'm not much help...:sleep:
 
#3 ·
I think a 305 will fit on a gt4 rim. On my cayman I went from a 235 to a 245 up front, and while the tires were thicker, it was fine. 10MM won't change a fitment issue. Especially considering that the GT4 rim looks like it could hold more tire in the rear. FYI this is my humble opinion. I may be completely wrong.
 
#6 ·
A 305/30 absolutely will fit. Porsche does both 295/35s and 305/30s on 11.5-inch wide rear wheels on 911s (which you get depends on the wheel) as factory fitment.

As you may have noticed, Porsche does run a smaller profile on the wider section width, as the overall diameter does increase a teeny bit when moving from a 295 to a 305 on a 11.5-inch wide wheel (would be a different story with a 12.0).
 
#7 · (Edited)
Just posted this on another thread so it's still on my clipboard.....If you look at this site and input your wheels size and offset, you can see for different tire sizes and aspect ratios what the effect is on clearance and speedometer error. I swear by The Car Bibles when it comes to tires...

https://www.carbibles.com/tire-size-calculator/
 
#9 ·
I see that you are talking specifically for track so our goals are likely different, why the desire to increase the size?

My plan for my 718 Spyder is stepping up one size larger on the Rear OEM wheels, the 20x11 wheel will accommodate the tire without any problem and the additional section width and tread with is minimal however... I have yet to put this combination on the car. Also be aware that the tire brand does determine the tread/sidewall profile, what tire are you looking at? Is this a 981 GT4?
 
#12 ·
FWIW, I have the 305's on the way for the rear, still waiting for the 245's to come back into stock... I hope this means that these are all fresh tires! :unsure:
 
#13 ·
I drove the 718 Spyder home from the dealer today. There were no problems with the 305 Michelin 4S tires on the rear. I did not push hard since I was just taking delivery (G-meter only up to .7) but no untoward noises. I only drove 60 miles since there is no PPF until Monday, and no registration until whenever.

i was surprised to see the water temperature at 220F and the oil at 240F with no Sport mode to reduce them. I will have to determine if these are normal.

williamr
 
#14 ·
Thanks for the update, all of the tires have now shipped, just a waiting game now...

So I don't recall the oil temperature but can confirm that the water in my Spyder sits at 105C which is 220F...
 
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