Macster

Are they the same tires? Are you sure the hardness and tread wear numbers are the same?

What else then can explain the behavior that prompted you to post asking about it?

Also, you have to consider the tire quality control may be lacking. The tires are quite cheap compared to N-rated tires. Perhaps you do get what you pay for? Or in the case of the new rear tires, you don't get what you don't pay for.

All I know is I've run uncounted sets of tires on my Boxster (and a number on my Turbo) and when replacing just 2, or all 4, sometimes with another brand, I've never had the car exhibit or manifest any odd behavior. The cars feel the same, drive the same no matter if I have Pirelli or Michelin, Continental or Bridgestone tires on.

996tnz

I researched the Sumitomo HTRZIII quite a bit before buying them. If it helps anyone do the same a bit quicker, here's how I've found them so far.

Been daily driving my 996TT for the last year on a set of Sumis so about 5000 highway miles, 3000 suburban miles, 300 'canyon' miles and over a hundred tight track miles including sprint competitions plus a gymkhana on a super rough airport runway that tore big chunks out of many other car's tyres but just scuffed mine heavily by a mm or so.

I run pretty aggressive camber (for a stock TT) and at my last DOT inspection (WOF) I was expecting them to be worn out or even starting to cord on the inside but the Sumis still had 6mm of tread left across 3/4 of the face. I like using my horses so am very surprised they're holding up that well.

Here's how I'd rate them:

GOOD:

Y rated to 300kph/~190mph

Squeally and progressive at the limit

Great in dry and still very good in wet

Fast on track (for street tyres)

Proper 996TT/C4 AWD rolling diameters*

Rear 295-30-18s have a heavy load rating - matches the 911 well.

Great wear (300 rated). Presumably helps minimise flat-spotting in a spin too.

Lower priced than most comparable tyres


NOT SO GOOD:

Flat spot when parked (annoying-takes 3-5 miles to come right when cold)

Get very hot when worked hard - 10 psi difference between hot track and cold pressures (on track I just start 5 psi low then bleed air between runs as they heat up).

They are asymmetric but not directional - so the tread pattern is reversed between the left and right side of the car. This only matters if you notice such things and it sits low on my priorities.

They are Sumitomo branded (Sumitomo do Falken etc too, but it matters to some)

They have rim protectors but like most, these only half work.

Bit greasy first 100 miles or so (others report)

295 is as wide as they get

Not available locally in all markets (had to import mine from the US)


*Rolling diameters front to rear suit the 996 TT/ Carrera4 tolerances (2% I think), with the fronts just a bit larger than the rears - as Porsche designed it so that the faster turning rears push a little torque to the front when cruising to keep the front diff at operating temperature.

For my usage, the Sumis are great tyres with acceptable trade-offs. I like them better than Pirelli P-Zeros and they feel on a par with the N-rated Bridgestone S02s I used to have on the TT (the Sumis possibly lapping a bit faster but looser). From what I've read, the Michelin Pilot Sports are probably the best all-rounder street compound tyres for the 996TT, but cost a chunk more.

996tnz

Lance, as I'll likely be replacing my rears in another 6 months or so, it'll be interesting to see if I get the same problem (please no).

Beyond the heat-cycling difference, the most obvious difference is that your new rears will have (11mm?) of tread versus your used fronts having maybe 5mm or so. My rough maths suggests this alone would make about a 1.5% difference in the relative front-rear rolling diameters, reducing or eliminating the intended preload on the front diff. Your first post indicates you drive your car hard. On your old - matched - set of tires, the smaller rears (smaller when new and even smaller due to faster wear) would have kept the front diff warm and ready to go with 5% or more of torque already going to the front. The new rear, worn front combo may mean there is no difference in rolling diameter, with the car responding differently to spirited driving as it is slower to throw torque forwards since the diff is not already at the intended operating temperature - making it slower to react.

The above is only 'informed conjecture' and I would love a Porsche engineer to drop by and explain it all with some response graphs etc. Some of what Porsche has done to the 911 lately (electric steering, deleting the handbrake!) I find stupid but going to a better controlled clutch based AWD as in the 997 provides better control of such effects and makes the car less sensitive to tyre sizes (albeit more complicated and maybe less reliable).

(If a Porsche engineer does drop by, maybe they could also advise how long it is safe to power slide a 996 Turbo before the front diff goes bang (if a risk) and what effect a series of handbrake turns have on the front diff).

996tnz

Quick update. Measured them myself and front are 4-5mm, rears 3-4mm.

Thinking about tires today reminded me to get wheel balancing plus a check on a slow leak in a rear tire. They found a large 6" by 2" bubble on the inside alongside the bead, and a small puncture in the tread nearby.

The tire shop said the bubble was caused by physical damage rather than any internal fault. So I'll keep my current 3 good tires as spares and get another set of Sumis in. In the interests of science I'll get the rears replaced first, drive it a while, and only replace the fronts if the car feels awkward with old fronts, new rears.

Lance 4c

Update, I changed the pressure to 31/32, upon doing so...even though I always check my pressure!...somehow my pass side front was high by several pounds. (some bonehead must have sneaked into my shop and over inflated the tire. ) I was at 34/42 earlier. They previously felt balanced at stock pressures.... Anyway, with the tire pressures in balance they feel normal. Doh! What are your cold tire pressures 996tnz?

nick49

If it still feels not 100% you may need new fronts to get the diameter matched as close as possible. I went through the same thing with 50%worn fronts and new rears. Car felt scary at speed. I run Htrz lls, love them, reasonable price, great in the rain, pretty sticky too. I did a recent pull to over 165mph and the car was rock steady at speed, hands off the wheel at 130+.

thomapa1

Lance - Maybe it is a characteristic of Sumitomos, but I had lousy stability at higher speeds with them as well when my tire pressures were ~34/42 cold. Now run around 31-33 ish cold and they feel lots better. Nowhere near as much wandering/tramlining.

Kevinmacd

Brings up a good question any opinions on inflation pressures?

996tnz

Hi Lance. Usually about 34/42 cold for street. about 31/38 cold at track, then I bleed down to 33/41 or so while hot on coming off track each time between runs. After several sprint sessions I once let them cool overnight to see the total difference - they cooled to 26/34 from 36/44 so 10 psi difference. I'll likely run a bit lower pressures out back next time as the back is looser than I'd prefer.

I need to play with lower pressures too I guess.

996tnz

Update - they checked the other rear at that time and found a similarly large bubble inside on the sidewall. I gave Sumitomo another chance since that set had once been subjected to a 70mph 360 spin on a dry track. The rears of my second set had premature tread separation (eg an inch square chunk coming off to reveal the cords) without such a spin so I'll be changing to something more durable since I'd never had tires fall apart on me before, never mind twice in a row. More info here if anyone wants it:

https://rennlist.com/forums/996-turb...?#post11833777

"02996ttx50

best starting point on the street is 36/40. then adjust downward -2lbs front and rear. assuming empty car light driver on the street even if you have some crazy streets as i do. 34/38-9 on the low end works also.

porsches door sticker numbers it must be remembered are for fully weighted cars toting people luggage and n/a lol