31 Inch Spare Tire?

[summerinmaine]

Now that I have 31/10.5/15s on my SWB MJ, I'd like to match them with a spare that's larger than stock.

 

Anyone know whether that tire diameter will fit into the stock spare tire carrier without fouling? I'll probably just mount the larger tire on the stock spare rim, rather than an 8" rim.

 

I know I can crawl under and measure, but it's late, and I'm lazy. :brows:

 

Thanks.

[JeepcoMJ]

Now that I have 31/10.5/15s on my SWB MJ, I'd like to match them with a spare that's larger than stock.

 

Anyone know whether that tire diameter will fit into the stock spare tire carrier without fouling? I'll probably just mount the larger tire on the stock spare rim, rather than an 8" rim.

 

I know I can crawl under and measure, but it's late, and I'm lazy. :brows:

 

Thanks.

 

worry more about the weight of the tire than the diameter.

 

you're not getting it to fit in there with the stock carrier, without having weight issues...it'll tear that cable lift to pieces.

 

so, bedmount.

[HOrnbrod]

I've got a 30" under there, and also have 31" tires. Fits fine.

[Eagle]

I carried a 31. Be sure to inspect the condition of the winch cable and the hanger. A 31x10.50, especially on a steel rim, is heavy.

[summerinmaine]

worry more about the weight of the tire than the diameter.

 

you're not getting it to fit in there with the stock carrier, without having weight issues...it'll tear that cable lift to pieces.

 

so, bedmount.

 

Bedmount is not an option with my camper.

 

I'll look into the weight issues.

 

Thanks

[james750]

i have a 31 with a steel rim under there no problem. I have a huge exhaust system on their though so i have to watch to make sure the tire doesnt rest up against the exhaust pipe. ;)

[summerinmaine]

I've been giving this thing a bit more thought, and what I'm looking to accomplish is a spare that has the same rolling diameter as the other wheels, so that I can run it in 4WD without stressing the diffs.

 

If the diameter of the spare fits in the available space, and weight is the only matter of concern, then I should be fine if I come up with any appropriate wheel/tire combo that has the right diameter, and matches (or reduces) the weight of the stock spare, right?

 

A narrow width spare on a narrow alloy rim, and possibly stored uninflated (since I always carry a pump) would be one possibility.

[Eagle]

Weight isn't a problem as long as your winch is healthy. I had my 31 on a factory steel rim and it stayed up just fine. But I've seen some of those assemblies where the "tee" part that fits through the center hole of the wheel was virtually a memory. Being in California, you may not need to worry about that.

 

Just using an alloy rim will save a few pounds. And the idea of a narrower tire is also a good one. You may be able to find a passenger car tire in an 85-profile that's pretty close in diameter. Otherwise, just inspect your winch and carry the 31.

[summerinmaine]

Weight isn't a problem as long as your winch is healthy. I had my 31 on a factory steel rim and it stayed up just fine. But I've seen some of those assemblies where the "tee" part that fits through the center hole of the wheel was virtually a memory. Being in California, you may not need to worry about that.

 

Just using an alloy rim will save a few pounds. And the idea of a narrower tire is also a good one. You may be able to find a passenger car tire in an 85-profile that's pretty close in diameter. Otherwise, just inspect your winch and carry the 31.

 

It's a California truck, so there's no rust or corrosion evident. I've looked at the "tee" and its connections to the cable, and everything looks solid. I'll probably lube the cable and the pivot well, so it doesn't bind up. But I'll probably still look to keep the weight as low as possible, just as a precaution.

 

I'll do the calculations and figure out what profile tire would approximate a 31" tire (according to my measurements, more like 30.5" on a 15X8 rim). Just guessing that a 265/75/15 should be about 30.75" in diameter, right? Then I need to figure whether that will actually reduce the weight all that much.

[Eagle]

265 / 25.4 = 10.43

 

That's not much of a difference. Try an 85-series tire.

[summerinmaine]

265 / 25.4 = 10.43

 

That's not much of a difference. Try an 85-series tire.

 

So, for a 265/75/15 tire the calculation for the diameter is:

 

10.43 X 0.75 = 7.82 (side wall height)

 

15 + (2 X 7.82) = 30.64" diameter, right? That's pretty close to the 30.5 that I measured for my nominal 31" tire.

 

Or am I screwing up my calculations somewhere?

[Eagle]

265 / 25.4 = 10.43

 

That's not much of a difference. Try an 85-series tire.

 

So, for a 265/75/15 tire the calculation for the diameter is:

 

10.43 X 0.75 = 7.82 (side wall height)

 

15 + (2 X 7.82) = 30.64" diameter, right? That's pretty close to the 30.5 that I measured for my nominal 31" tire.

 

Or am I screwing up my calculations somewhere?

Nope, you're not screwing it up. That's how you figure it.

 

Keep in mind, that's the unloaded profile. The best measurement is the "static radius" taken from the manufacturer's specs, or the revolutions-per-mile. Different tires have different sidewall stiffness, which affects how much a given weight will defect the sidewall. Several years ago, Cooper Tire offered the original Discoverer radial in a P-rated, passenger car version and in an LT light truck version. Same tread, same sizes. The static radius and revolutions per mile were different for the two tires in, for example, a 235/75R15.

 

But, for a spare, getting something that's within a quarter inch of the tire diameter will probably be as close as you're going to get.

[eacomanche88]

I have a 32-10.5-15 under mine on a Explorer steel wheel and fits fine. Its been under there for 2+ years now without issue.

[jaekl]

My owners manual mentions the use of a strap under the spare when a full size spare is used. Has anybody seen one of these? I would think it attaches to the frame on both sides as a precaution.

[summerinmaine]

My owners manual mentions the use of a strap under the spare when a full size spare is used. Has anybody seen one of these? I would think it attaches to the frame on both sides as a precaution.

 

I've often thought that would be a good idea.

[Tracker]

This was a little bit of work but I didn't have anything to put any spare on. It's hard to see the whole idea from this pic but it's a drop-down frame that holds a 32" tire. This was in the MJ trailer I made last year. It's mostly Uni-Strut scrap, kind of shaped like a peace sign, and it hinges down to get the tire out.