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Can you damage a rear end with a mini spare tire?

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Can you damage a rear end with a mini spare tire?

Old 11-25-2018, 02:57 PM
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Default Can you damage a rear end with a mini spare tire?

Bought a new toy and of course it doesn't have a spare. Just the air pump and fix o flat came with it. Even though I have a 60,000 roadside assistance on tires, I don't want to be left stranded. Call me old school, but I would like a mini spare tire for to get home if ever needed. I have 20 inch wheels and the mini spare tires seem to all be 18 inch and not recommended for cars with 3 other 20's on it. I know it depends on the overall height of wheel and tire, but there will be a diameter difference in order to fit the wheel well area in the trunk. It may never be needed, but can I go 50 miles or so at 50mph with an undersized mini? The websites including Ford say no probably for legal reasons, but the parts department guys at dealers say I would't have damage to the differential if on the rear, or other areas if on the front end. That may be a false statement. Who to believe? Am I worrying about nothing if I have a flat? What say you gear heads?

Not as fast as the Turbo S, but definitely fun and sounds perfect.
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Old 11-25-2018, 03:04 PM
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If you have a limited slip diff, you DON'T want to be running a different diameter in the back for any kind of distance. In that case, the proper procedure would be to put the spare on the front wheel and move the front wheel to the back, assuming that the front/back are identical. You may find that you have an open diff (with "electronic slip") in which case I don't think it matters.

If your wheels are staggered (which I assumed on a 5.0 they would be) the above trick would be more complicated and only worth doing if you're sure about the diameter.
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Last edited by Flot; 11-25-2018 at 03:31 PM.
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Old 11-25-2018, 03:22 PM
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You'd be fine, the reason you have a differential is because in a turn the inside wheel spins slower than the outside wheel and if both axles were connected you'd have all kinds of fun.. I don't see a reason to go 50 miles but a few miles isn't going to hurt anything. Limited slip or open, does not matter. Your tires are the same size at all 4 corners. Don't make it harder than it needs to be.

Edit to add that you would be much better off having the spare on the back. You want as much traction up front as you can get since the majority of braking and all the steering happens up there.
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Old 11-25-2018, 03:26 PM
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1 wheel size does not matter
2 its the size of the tire
3 you can always swap from front to back if needed for distance
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Old 11-25-2018, 03:34 PM
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Originally Posted by Lookinup76 View Post
You'd be fine, the reason you have a differential is because in a turn the inside wheel spins slower than the outside wheel and if both axles were connected you'd have all kinds of fun.. I don't see a reason to go 50 miles but a few miles isn't going to hurt anything. Limited slip or open, does not matter. Your tires are the same size at all 4 corners. Don't make it harder than it needs to be.

Edit to add that you would be much better off having the spare on the back. You want as much traction up front as you can get since the majority of braking and all the steering happens up there.
. This is all wrong. See post #2 above. For a rear wheel drive put it on the front, drive slow and get it fixed asap. For front wheel drive put it on the rear, drive slow and get it fixed asap
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Old 11-25-2018, 03:44 PM
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Probably has a posi rear end. DON'T do it with a posi!!!!
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Old 11-25-2018, 03:44 PM
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Originally Posted by xl883lo View Post
. This is all wrong. See post #2 above. For a rear wheel drive put it on the front, drive slow and get it fixed asap. For front wheel drive put it on the rear, drive slow and get it fixed asap
I respectfully disagree and stand by my original statement. What is more important stopping and steering or forward acceleration? In an emergency or panic situation the limited traction of a donut spare can get you into serious trouble if it is up front.

I agree 100% with slow and limited distance.
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Old 11-25-2018, 03:52 PM
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Nice looking car! Mustang right? Year?
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Old 11-25-2018, 04:02 PM
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Originally Posted by tcontic View Post
Nice looking car! Mustang right? Year?
thanks. yes 2019 GT Premium. absolutely love it.
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Old 11-25-2018, 04:19 PM
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A a a....
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Old 11-25-2018, 04:30 PM
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Uh you probably have run flats you can go farther on those than a spare. I guess if you think your tires or wheels are going to fall off you can put one in the back? At that point I wouldn't be worried about transmission damage lol
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Old 11-25-2018, 04:33 PM
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Is there even a jacking point on the car??
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Old 11-25-2018, 04:34 PM
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Originally Posted by Lookinup76 View Post
I respectfully disagree and stand by my original statement. What is more important stopping and steering or forward acceleration? In an emergency or panic situation the limited traction of a donut spare can get you into serious trouble if it is up front. I agree 100% with slow and limited distance.
With something like that 5.0 Mustang with Traction Control and a Limited Slip Differential it is not designed for the computer and the mechanicals to accommodate a donut spare. Hit a bad spot ( rain, oil, crack sealer), or get into a curve a unexpectedly fast and you will end up in the ditch quicker than you would ever imagine. Just a couple psi difference in the rear tires will cause one of those 5.0 Mustangs to noticeably pull to one side. Add to it that with the LSD you may get premature plate wear or even heat the rear end up enough to cook the fluid (this of course depends on the road and how long and fast you drive it). Don't do it.
There are RWD vehicles that it may be better to put it on the rear, like a 1980 Chevy Chevette, but a 2019 5.0 Mustang is not it.
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Old 11-25-2018, 06:53 PM
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What you should be concerned with, will the doughnut fit over the Brembo brakes. (Brembo Calipers or the 67G Performance Package).Does your car have 20 inch wheels? If so doughnuts are not recommended.

Don't forget to buy a jack and lug wrench
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Old 11-25-2018, 06:54 PM
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Buy a plug kit and a mini compressor. No chance I'd put a donut on that car, especially on the rear. You'd be lucky if the body clears the ground anyways and that's before you blow the donut and tear up the fender.
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Old 11-25-2018, 07:26 PM
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This is what the owner's manual for my 98 Trans Am with the limited slip differential rear axle says:

"If the compact spare is used as a rear tire, do not drive faster than 50 mph (80 km/h). Damage to the rear axle may occur if the compact spare is driven faster than 50 mph (80 km/h)."

No mention is made about switching wheels front to back in the manual.

I doubt that the Mustang rear axle is inherently weaker in this regard. But, personally, I would stick with the supplied pump and repair kit. My compact spare has never seen the road in 20 years.

Last edited by GaryDoug; 11-25-2018 at 07:34 PM.
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Old 11-25-2018, 07:42 PM
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This may sound crazy, but if the spare doesn't work out, don't sweat it.

Just pull over and take the flat tire off and then just drive on 3 wheels. No joke at all.

I mean I wouldn't try and race at a redlight or drive like a nut, but I would totally drive on the highway or whatever to get home or to a tire shop.

I drove almost 100 miles on Highway 101 and then surface streets with only 3 tires once to get home. The wheel had just fallen off. I could totally turn corners and everything and the truck was fine. It was a rear tire/wheel. Don't know if it makes a difference front or back.

From that S to a Mustang huh? Shes a great looking car, but man that Porsche was sexy.
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Old 11-25-2018, 07:49 PM
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Trucks are way stiffer than a Mustang ;-)
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Old 11-25-2018, 08:56 PM
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I hope it lasts longer then mine. I had the 10spd that went out twice in 4K miles. Then would randomly lock up in the 3>4 shift.

Other then trying to randomly kill me. I loved the car.
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Old 11-25-2018, 08:57 PM
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Other then that there was a company who made the 20” doughnuts to clear the performance pack breaks. I forgot the name of them now though.
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