Spare Causing Differential Damage

[Jinx]

I finally have my tire on order and my powdercoater picked for my wheel set; but one issue that has been bugging me is losing my spare.

I'm going to 6 piston Brembos and rear 2013 GT500 rotors. So the OEM spare is useless now. So I've been thinking of picking up a 19x8.5 GT500 2013 style wheel from AM and throwing on a cheap 255/40/19 on there for a spare. That will fit in the spare tire well (it will stick up an inch).

Problem is that I read that any tire/wheel height difference of more than 3% between two rear tire/wheels can cause damage to the rear differential. Now I know a spare is a temporary measure; but just how fast or type of conditions can make this damage come about?

 

[eighty6gt]

If you were to drive in a circle that's about 30 feet in diameter you have around 10% difference in your rear wheel speeds, which is the same as if you had a 3% height difference (all gorilla math here.)

Wear in a clutch style diff will occur immediately, in a fairly linear fashion. I'd just say you'd take a year or two of life out of the assembly limping 100 miles home.

Finally, the OEM diffs are terrible anyway, just save up and get a trutrac, I think they would mind this sort of thing a lot less. I haven't spoken to Eaton about tire diameters, would be a good thing to check into.

 

[Shotokan1509]

Put the spare on the front and swap a front to the rear to replace flat

 

[claudermilk]

Put the spare on the front and swap a front to the rear to replace flat

Oh there you go thinking & being logical...

 

[TGR96]

Put the spare on the front and swap a front to the rear to replace flat

 

[Kylar]

ill go against the grain and say that driving home on that wheel wouldnt cause any issues. Driving for a few months might take some life out of the clutch packs.

 

[Kylar]

Put the spare on the front and swap a front to the rear to replace flat

I think hes saying the spare wont fit on the front or rear, because both are upgraded brakes. Unless he gets a gt500 spare.

 

[fast Ed]

No such thing as a GT-500 spare, they all came with a mobility kit in the trunk.


Driving home in an emergency situation with a slightly different tire size isn't going to kill your diff.

What tire sizes do you have on the car now? If they are a bit taller, go with the 245/45R19 non-Brembo size for putting on the aftermarket wheel to be used as a spare, that will get you closer.


cheers
Ed

 

[Jinx]

I want a spare, thats why I want a wheel that fits in the spare well.

The plan:

255/35/20 : Fronts

305/35/20: Rear

255/40/19: Spare


The rear is the issue. It's still over 4% difference in diameter using a Front tire and Spare Plan.

 

[oldVOR]

Run a 295 square setup and you won't have any issues.
Swap a front to the rear and spare on the front end.
You'll appreciate the additional front grip from the 295 with virtually no loss out back stepping down 10mm.

 

[retfr8flyr]

Just get a GT500 inflate kit, what are the chances of actually needing a spare. I haven't had a spare for 9 years, for the same reasons you have. I have the GT kit and a hand tire plug kit. If I ever have a flat and the hole looks like it's too large for the sealant then I will stick a plug in it, to get home. I haven't used it yet.

 

[Jinx]

Run a 295 square setup and you won't have any issues.
Swap a front to the rear and spare on the front end.
You'll appreciate the additional front grip from the 295 with virtually no loss out back stepping down 10mm.

Would be nice, but I dont think the rear wheels will clear the 6-piston brembos

 

[Jinx]

Just get a GT500 inflate kit, what are the chances of actually needing a spare. I haven't had a spare for 9 years, for the same reasons you have. I have the GT kit and a hand tire plug kit. If I ever have a flat and the hole looks like it's too large for the sealant then I will stick a plug in it, to get home. I haven't used it yet.

Sigh. I guess the answer to my question. You can't have a spare if you want one with a 305/35/20 rear tire.

 

[TexasBlownV8]

I disagree on using just an inflate kit. MOST of the tire incidents I have had when driving have been on highways or on similar roads, and most have resulted in complete destruction of the tire, or a big hole by the time the car is stopped. THEN what are you going to do?!!
I suppose "statistically", most tire issues are not that dramatic, such as you come out to the car and a tire is flat, and it can be re-inflated with an inflate kit. Yes they are useful for that.

I prefer to have a spare of some kind, so that if you must change the tire, you don't get stranded..or have to wait for AAA or whoever.

Go for a narrow tire vs wider one. Or if you are about to replace worn out tires and have a suitable extra rim, have your least-worn tire be used as your spare.

 

[Jinx]

Where would you put a spare with a 28.41" tire height?

It would have to be just sitting in the trunk instead of inside the spare well

 

[Racer47]

Just do both. Keep a can or 2 of fix a flat and some tire plugs in the trunk to use for small punctures. Use the spare tire size you listed. Keep that tire at about 50 psi or so. If you really need it for the rear, let some air out of the other rear tire, down to maybe 25 psi at the lowest. This will close up the gap in tire diameter some. Then just drive slower and dont build a lot of heat in the diff. The diff will live long enough. I think you are too worried about this.

 

[Jinx]

Just do both. Keep a can or 2 of fix a flat and some tire plugs in the trunk to use for small punctures. Use the spare tire size you listed. Keep that tire at about 50 psi or so. If you really need it for the rear, let some air out of the other rear tire, down to maybe 25 psi at the lowest. This will close up the gap in tire diameter some. Then just drive slower and dont build a lot of heat in the diff. The diff will live long enough. I think you are too worried about this.

I wasnt until I heard the damage that could be done in just a few miles.

How much lower in tire diameter at 25 psi?

 

[Racer47]

How much lower in tire diameter at 25 psi?

I dont know but its not as much as you may think based on how much tire deflection there is at 25 psi. The belt package is stiff and still dictates most of the effective rolling radius.

You can measure it though. Mark the tire and ground, drive the car forward slowly, have someone count 10 revs, stop with tire mark back on the ground, measure distance, calculate rolling radius.

A few miles wont kill your diff. Its not exactly the best thing for it but it is designed to slip.

 

[Norm Peterson]

I want a spare, thats why I want a wheel that fits in the spare well.

The plan:

255/35/20 : Fronts

305/35/20: Rear

255/40/19: Spare


The rear is the issue. It's still over 4% difference in diameter using a Front tire and Spare Plan.

So why not consider 305/30-20's for the rear tires?

27.2" tall vs 27.1" for your 255/40-19 spare (Michelin PSS data) and still a tiny bit taller than the front 255/35-20's at 27".

An even better approach would be to match the revs/mile of your spare as closely as possible to the revs/mile of a set of 305/30-20 rears and let the revs/mile error fall mostly against your normal front tires, which don't have a differential to be concerned about.


Norm

 

[Jinx]

Cost. 305-35-20 is readily available and I already bought my Cooper RS3-S setup. It would put me over $1k for a street tire. Not desirable.


Sent from my iPhone using Tapatalk