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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.
 

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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:
 

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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.
 

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Thanks for the input guys. I think the only way to know for sure is to get one mounted and try it. It may be a while before I try because my current Cup2's have at least 50% tread left. Once I do I'll post the results.
 

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Maybe you can have the dealer mount up someone old 305 off a 911 or GT3 and see how it works before ordering. You can have them do the work to get the tire business. Will cost a little more cause they'll be a bit more expensive, but you will know for sure before you drop $500 plus per tire.
 

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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).
 

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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/
 

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Anyone tracking a GT4 with 305 30 20” on stock rims with yellow calibers? Do they fit OK?
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?
 

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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?
It looks like the 305 tire will fit. I had them mounted on my 718 Spyder today in place of the 295. I have not driven the car on them yet, as it just arrived at the dealer yesterday but the tech who mounted them did a short drive and said there were no problems. I went with the larger rear size as I wanted to replace the Cup tires and the only N Spec 4S available were the larger size. I see now that for the 2021 Spyder, Performance Summer tires can be speced instead of the Cup tires, but I have no idea what they actually use.

williamr
 

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It looks like the 305 tire will fit. I had them mounted on my 718 Spyder today in place of the 295. I have not driven the car on them yet, as it just arrived at the dealer yesterday but the tech who mounted them did a short drive and said there were no problems. I went with the larger rear size as I wanted to replace the Cup tires and the only N Spec 4S available were the larger size. I see now that for the 2021 Spyder, Performance Summer tires can be speced instead of the Cup tires, but I have no idea what they actually use.

williamr
@williamr Thanks for this follow up, much appreciated, I have been trying to order some UHP All-seasons for my Spyder and want to use the 305 sized Bridgestone's on the rear because they run a narrower tread then most other tires of the same size. Those N-spec 4s in the 305 are monsters, same section width and height but the tread is nearly an inch wider that the 295 size! If you car is not complaining then mine should be fine!

Please let me now if once you get behind the wheel of your car the are any issues with the 305's out back.

BTW, Congrats!
 

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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:
 

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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
 

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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
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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