redpathtribe

Cross posting this from the Tire & Wheel Forum...

I need some tire loading education and help please. I have a '90 928 GT with OEM Porsche 19" Carrera Classic wheels off of a Cayman installed. Love the looks and have had them on the car for a while with no issues.

I'm in the process of changing out the rear tires to match the fronts, Michelin Pilot Super Sports. Rear wheels are 9.5"x19" et46. I have 265/30-19's on them right now and want the same size to try and keep the overall diameter close to original size for a 928 (i.e. close to 25"). I ordered the 265/30-19's from Discount Tire, but now they are saying they won't/can't mount them because their "new" computer program says my 928 needs a tire with a load index of 94 (1477 lbs.). The 265/30-19 Michelin Pilot Super Sports have a load index of 93 (1433 lbs.)

Here's where I get lost... The tag on the door jamb of my 928 says rear axle load is 2425 lbs. (2) of the Mich PSS's at 1433 lbs. each would be a load rating of 2866 lbs. which is obviously greater than 2425 lbs. rated for the car. Is there an additional safety factor for the loading that would make the PSS's inadequate in this size for my car? Am I missing something here?

One last part of the frustration... the tires currently on the rear are Kumho Ecsta SPT's in the exact 265/30-19 size that the PSS's that I want to put on. They have the same load index (93) as the PSS's, and Discount Tire installed them!!! They said they know that, but that was before their computer system update, and they can't install that size now.

Thanks for listening, any help in educating me where I'm not understanding the problem would be greatly appreciated.

jcorenman

Rick,

I ran into the same problem with Discount Tire a year ago with PS2's, story here: https://rennlist.com/forums/928-forum/760823-dunlop-direzza-no-longer-allowed.html#post10536438

I wound up switching to PSS for all four. Great summer tire, but this load-rating business is annoying. I got the same 94-or-greater story, I don't understand it, but at least they are consistent.

RKD in OKC

Not an expert, but here is my impressions..

Porsche tire specs used to require Z rated tires not for the top speed as most assume, but to insure the load rating was high enough for handling performance. This is what I was told ten plus years ago when purchasing tires for my 944 Turbo. In fact, back then the Formula one tire guy at Bridgestone told me he road raced and would not use rims bigger than 16 in because lower profile tires would not provide rim protection at speed. Tires have changed since then as newer cars are coming out with lower profile wider tires from the get go, hence the move from Z or N rated tires to the Load Index rating. Think you will find the Z and higher speed rated tires typically have a load index of 94, especially in the OEM tire sizes.

Jon B.

Why not just tell them they're going on the Cayman the wheels came from? I'm all for being honest, but in this instance you're getting the tires installed hassle free if you tell them what they want to hear.

Doing the "tango" when you've done your due diligence on the tire research is just a hassle. I never have them mounted from tire rack, they send them to me and they're installed at a shop of my choosing. Much easier and my shop never asks questions.

RKD in OKC

The different Load Index ratings have almost the same lb capacity at 35-36 psi, the higher rating increases weight capacity more than the lower index ratings as both temperature and tire pressure increases.

Alan

Rick,
Just take the tires to Wheel Specialists in Tempe to mount them - or buy them from Tire Rack and have them drop shipped to them. Have done this many times.

Mounting is a little more expensive (a little) , but these guys are good - there is always a Porsche, Ferrari or Lambo there when I go - they take extra care, they torque everything to spec by hand, and lift by hand. If you need wheels straightened, widened or refinished - they are very good.

Alan

redpathtribe

Jim, thanks for the link, missed that thread and as you noted, I got the same story that they were now going off the load rating of the original tire, not the actual load rating of the rear axle. Doesn't make sense to me unless there's another % safety factor or something I don't know about.

The Michelin Pilot Super Sports in question are Y (186 mph) rated tires... and a 93 load index. I think the load index has more to do with the size and section of the tire.

Problem was they already have me (and my 928) in their computer system. I've bought many tires from these guys. I did say I'd just bring the wheels back loose and have them install the tires, but they said they now know what the tires are going on and can't install them.

Alan, that's an option I'm considering. I could also solve it by going with 275/30-19's, but with the et46 offset on the 19x9.5 wheel, the 265's are REAL tight against the factory rolled GT fender. Just sounds like excess liability has creeped in here somewhere and I'm being protected from myself...

Alan

Get what you want - don't let these guys dictate to you.... it is smart that they warn you, and possibly even refuse to sell something that is clearly way off. For the other cases like this you'd think they would just have you sign a waiver... just how hard are they trying to **** their customers off here!?

Alan

17prospective buyer

You have to pay extra to get that "take your time" service...

redpathtribe

Victory!!! Went to another Discount Tire in north Scottsdale and bought the tires, brought in the wheels loose, and had them mount the new tires on the wheels no problem. Just took them back home, cleaned everything up and mounted them on the rear of the GT. Gee, I hope they work OK... They knew exactly what I was doing and why, and were just as frustrated with their liability minions as I was.

The guys at the store told me they have a lot of "fun" with the tire loading index they are stuck with on their computer system. Apparently basically identical vehicles like a Yukon and Escalade can't have the same larger tires and wheels installed...

Now will be very interested to see how well the Michelin Pilot Super Sports wear, one of the reasons I got them was their reported excellent (relatively) tread life. The Kumhos I replaced gave me 25k miles, not bad for the rears, high performance summer tires, and somewhat of a heavy foot... I will say the right rear was a "little" past the wear bars...

Alan

Rick since the PS2 sizes on 17" are getting more limited I'll probably be looking at the Super Sports next time. Interested to hear what you think of these after they have been worn in a little.

Alan

Speedtoys

The problem with waivers, is that they're useless.

They don't protect you from someone suing you and typing up funds and bad press.

If they say no..maybe they pissed off 10 people, total. But it never made the news either.

Alan

Surprisingly not that much - maybe $50 for 4 - they make their money on selling wheels, refinishing wheels, straightening wheels. They are happy to do little jobs like fitting & balancing to get you as a customer.

There may not be so many places like this - but they are out there.

Alan

jcorenman

We fit Michelin Pilot Super Sports (18") on the GT at 219K in June last year, now 235K and the rears are maybe 2/3 gone... might make 25K miles. The PS2's which preceded them went 30K miles, but well into the wear bars. I think these are as good, maybe a little better, in terms of handling and traction.

Alan

Thanks Jim - reviews & ratings on TR seemed quite positive, good to hear the long term (miles) view too.

Alan