Riddle me this - making up a spare for a Perf Pack GT

[Spart]

I'm thinking about getting a PP GT, but I'd want to take the car on some long trips and I wouldn't feel comfortable without a usable spare - even if it doesn't fit in the well in the trunk.

There are a number of issues with getting a spare on the PP.

• Just buy the factory spare, duh
• Why it won't work: Doesn't fit over front brakes.​
• Okay smart guy, buy the factory spare and if the front gets a flat, swap one of the rears to the front then put the spare on the rear.
• Why it won't work: Two reasons, the rear wheels are staggered and won't fit properly on the front. The factory spare tire is also a different circumference from the larger PP rear wheel, and running different size tires on the rear could/will ruin the torsen diff.​
• Okay let's use some brain power. Let's say you make your own spare with an asymmetric tire that has the same circumference as the PP rear tires (so it can run on the rear without ruining the torsen) and the same backspacing etc. as the PP front wheels.
• Why it won't work: I don't know... can anybody think of why?​

So I did a bit of math and research. 245/45R19 tires generally will have the same circumference as the 275/40R19 tires that come on the rear with the PP, but in practice this will vary from manufacturer to manufacturer.

Pirelli makes a 245/45ZR19 that has a listed diameter of 27.6"

Pirelli appears to make more than one P ZERO in 275/40ZR19. I gleaned from the internet that what comes on the PP is the 101Y - of which Tire Rack maddeningly lists two. One made in Italy that's 27.5" in diameter and another made in Mexico that's 27.8". I want to be optimistic and say the PP rides on Italians, but it would make more sense that it runs on the Mexican made tires. Does anyone know for sure? Is there a even third P ZERO 275/40ZR19 101Y not listed on Tire Rack with yet another circumference?

Let's say for argument's sake that the mustang has the 27.8" diameter Pirellis in the rear. Is 27.8" vs 27.6" enough to damage the torsen? I assume that there is a certain range of difference that is allowable, going beyond which will cumulatively result in damage depending on the difference.

If that answer is no, it won't cause damage - is there any reason (rubbing) why this tire and a wheel close in size to the front PP wheel wouldn't work on the back?

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[Need4SpeedMotors]

Working on a spare wheel design already = ) information will come out shortly.

 

[RSPEC-015]

Some good info in this thread from the Australian Local Section

http://www.mustang6g.com/forums/showthread.php?t=41050

It's a bit long but if you zero in on posts by GNOME he sorted it out for us over here.

 

[Toms]

buy one of my 17 inch wheel from the v6 as your spare lol

 

[Spart]

Working on a spare wheel design already = ) information will come out shortly.

Awesome! Having a good answer to this will drive my purchasing decision so I hope "shortly" means "shortly."

Some good info in this thread from the Australian Local Section

http://www.mustang6g.com/forums/showthread.php?t=41050

It's a bit long but if you zero in on posts by GNOME he sorted it out for us over here.

I read pretty far into that - apparently Aussie Perf Pack cars do NOT get a torsen differential like the US market ones do, so matching the tire diameter for the rear may not be as important?

buy one of my 17 inch wheel from the v6 as your spare lol

You may have missed the point of this thread, your 17" wheel won't fit over Perf Pack front brakes.

 

[Toms]

Awesome! Having a good answer to this will drive my purchasing decision so I hope "shortly" means "shortly."



I read pretty far into that - apparently Aussie Perf Pack cars do NOT get a torsen differential like the US market ones do, so matching the tire diameter for the rear may not be as important?



You may have missed the point of this thread, your 17" wheel won't fit over Perf Pack front brakes.

This come to the old trick of fitting your rear to the front blah blah blah. You do realize that no matter how good the spare tire kit would be, you could never get the width and circumstance of your original tire right?
I know there are people out there who managed to run 4 19x9.5 gt pp rear wheels so putting one of the rear wheel front is doable. Spare tires are about temporary fix and it can get you from point A to B without much hassle and thats about it.

 

[Norm Peterson]

I'm thinking about getting a PP GT, but I'd want to take the car on some long trips and I wouldn't feel comfortable without a usable spare - even if it doesn't fit in the well in the trunk.

Trying to work with a third tire size seems like a whole lot of extra trouble for no benefit.

Why not just buy another 19x9 front wheel (or any 19x9 wheel in something like +42 to +45 offset, for that matter), mount a 275/40 on it (9" is tire industry acceptable for this size), and don't worry about putting that on whatever corner that the tire went flat on.

No matter how fussy you get with wheel revolutions per mile (which is a better guideline than outer diameter, BTW), you're still going to have a slight mismatch that will vary as your normal road tires wear. From new to the wear bars is going to be nearly half an inch diameter loss (and some uncertain increase in the revs/mile), while your spare wouldn't wear much at all even if you needed to use it a number of times.

For temporary use, the small differences in width here (255 vs 275) won't matter, and the non-driven front wheels aren't too sensitive to slightly different diameters (really hard braking might pull a little toward the side with the smaller diameter tire until the ABS gets interested in the proceedings).


Norm

 

[Shift]

Working on a spare wheel design already = ) information will come out shortly.

Paul! Please tell us you're working on the GT350 version also? Paranoid as hell with taking a road trip without a proper spare, and don't want to make a one-off as others have been doing.

 

[Spart]

This come to the old trick of fitting your rear to the front blah blah blah.

Again you miss the point of the thread and I think you didn't read the OP. There are two reasons why that's a bad idea and I mention them there.

Trying to work with a third tire size seems like a whole lot of extra trouble for no benefit.

There's got to be an acceptable range of tire size difference for a torsen diff, and I want to figure out what that is.

I can't imagine a tire that's 0.2" different in diameter will matter much. I mean, when you're hitting curves hard the diff is working all the time to some extent. I would imagine that heat is the main concern. So long as the heat is being dissipated, you should be fine.

 

[Spart]

Okay, coming up with some more ideas.

First of all, I found this thread which seems to indicate that small percentage differences that lots of people seem overly worried about won't hurt the Torsen diff: http://www.mustang6g.com/forums/showthread.php?t=40727

Additionally, a Race Star 18x5 drag wheel (92-850145DP) will apparently clear 2015+ PP front Brembo brakes, at least according to Lethal Performance. These wheels have -19mm offset.

Assuming you can fit a temp spare to a normal wheel, a T165/70D18 temporary spare will generally run about 2% smaller than the P275/40ZR19 rear tire size. Running 2% smaller has the same effect on the diff as constantly taking a turn with a 275 foot radius - think interstate cloverleaf interchange rated at 35mph.

A P185/65R18 tire will generally run 0.8% smaller than the P275/40R19 rear tire size. Running 0.8% smaller has the same effect on the diff as constantly taking a turn with a 688 foot radius - think a long sweeper rated 60mph+.

I'm liking the P185/65R18 idea. The range of wheels for that tire size is apparently 18x4.5 to 18x7, so the 18x5 drag wheel is just barely on the low end of that range.

It's 0.3" difference in diameter too. When the factory Pirellis are getting worn they'll be about the same size.

I also wonder if this setup would fit in the spare tire well. It's 7.3" wide and 27.4" in diameter. I tried to find the dimensions of the spare tire well using the forum search and Google and failed. Anybody want to find a tape measure and fool around in their trunk for five minutes? I'd be hugely in your debt!

 

[TexArmageddon]

.... Here's my suggestion... Buy run flats... Stick your none run flats in the trunk. LOL

 

[Norm Peterson]

There's got to be an acceptable range of tire size difference for a torsen diff, and I want to figure out what that is.

I can't imagine a tire that's 0.2" different in diameter will matter much. I mean, when you're hitting curves hard the diff is working all the time to some extent. I would imagine that heat is the main concern. So long as the heat is being dissipated, you should be fine.

From that other thread, it sounds like the ABS/TC/AdvanceTrak systems need the difference in revs/mile to be less than what the Torsen can at least tolerate.

That said, I'd suggest not using heavy amounts of throttle (particularly while cornering) when the drive tire diameters are known to be different.


Norm

 

[Spart]

.... Here's my suggestion... Buy run flats... Stick your none run flats in the trunk. LOL

I know your comment is in jest, but never again! I had two JCW Mini's that came with run flats, and they were ditched for conventional tires on both cars. I feel like they're a scam. They ruin the ride, and for what? You get to drive 40 miles at 40mph on a $400 tire that is ruined after said 40 miles so you can pay $400 again? No thanks.

I carried a full-size spare in the R56 JCW on long trips (which also had the problem of needing to fit Brembos) but with the rear seats folded flat that car has heaps of room for that sort of thing.

From that other thread, it sounds like the ABS/TC/AdvanceTrak systems need the difference in revs/mile to be less than what the Torsen can at least tolerate.

That said, I'd suggest not using heavy amounts of throttle (particularly while cornering) when the drive tire diameters are known to be different.


Norm

I'd be a bit worried about how the car's dynamics are affected. Cornering hard on a narrow rear spare is asking for it. My concern would be to find a decent tire shop without needing to be towed there, that's all.

 

[Spart]

So I bought a car with the PP... [MENTION=11685]Need4SpeedMotors[/MENTION] any updates on your spare solution?

 

[Norm Peterson]

I'd be a bit worried about how the car's dynamics are affected. Cornering hard on a narrow rear spare is asking for it. My concern would be to find a decent tire shop without needing to be towed there, that's all.

That's why I'd spec the spare to match the rear tire size rather than the front, but in the case of an S550 GTPP probably mount it on a front size wheel for reasons involving inboard clearance. No drive asymmetry other than by differences in wear, and any minor front asymmetries are more easily dealt with because you have direct steering control over those wheels.

I figure if the flatted tire is non-repairable, I'm in it for at least one new tire, and I wouldn't want to be buying one oddball make tire to go with the 3 MPSS or 3 GY Asymmetrics because that was all the shop had on hand . . . and then have to buy one or more matching tires anyway. If it is repairable, I can plug it myself. Either way I want a spare fully capable of getting me home without placing me under too much restriction.


Norm

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