Ram 3500 - Spare tire options for 37s.

[IdaSHO]

That's actually incorrect. The speed, regardless of PSI will always be the same. Because the tire from point to point, if you use the same reference, will always cross the same amount of distance.

flatten a tire in a rig with a locked diff on pavement and report back.

 

[p nut]

Perhaps I should clarify, as there are obviously few ways to look at this.
I purposely omitted the word radius for a reason.

With a lower PSI tire the distance (okay... .radius) from ground to center is different.
But thats the point. No way, no how is that irrelevant.
That difference is the perceived change in circumference the wheel sees.

And inflation for ABS/wheel sensor based TPMS systems are FAR more sensitive than some in this thread would think.
They do not need a severely deflated tire to trip the system. They can be, and are accurate to a few psi

This is what makes sense to me in my head.

 

[Kingsize24]

I have, many times, and on multiple vehicle types, the outcome is the same.

What you are describing is aspect ratio, not radius.

 

[p nut]

I have, many times, and on multiple vehicle types, the outcome is the same.

What you are describing is aspect ratio, not radius.

But aspect ratio is part of the “radius.”
ie On a tire size, the aspect ratio is the middle number (275/70/18). 275mm tread width, 70 is the aspect ratio (to the tread) and 18 is the rim diameter.

 

[Kingsize24]

But aspect ratio is part of the “radius.”
ie On a tire size, the aspect ratio is the middle number (275/70/18). 275mm tread width, 70 is the aspect ratio (to the tread) and 18 is the rim diameter.

That is true, but aspect ratio is what is affected when pressure is reduced.

A higher aspect ratio means a taller sidewall, and higher or lower pressure will determine effective aspect ratio, which is why this isn't mathing for you.

 

[IdaSHO]

To be fair, NONE of this even has bearing on the original question…. does a deflated tire have a smaller diameter, in order to fit in the stock spare location.

The answer to that is yes…. and no. Without weight on it, a tires diameter varies very little. However, deflate it enough and it will deform to fit…. but thats not advisable.

But when weight is added…. things get dicy.
This topic has likely been beaten and battered a dozen times over online….
But until its actually been addressed in practice (myth-busters…. hello!!??)
who really knows whats what.

 

[Kingsize24]

It's definitely off topic. Lol

 

[Transit720]

You'd likely have to dump your exhaust in front of the axle or totally reroute it...when I tried to stuff a 37 under my 5th gen the hitch and the exhaust were going to be the issues.

No need to modify exhaust on a 5th gen ram with flatbed and custom (larger) hitch. I can send photos when I get some time.

 

[renottse]

And for clarification to the OP my 37” tires are 36.5” diameter measured, (BFG KM3 and KO2) so no squeezing, man handling, 10lb sledge, swearing or deflation required… a true 37” is going to be the same in to in as the rails, others will have chime in on what brands they can fit in there.

 

[rruff]

Simple geometry.

And incorrect.

I used to believe this as well, but the really simple geometry is what I stated earlier. There is a steel belt under the tread that stretches very little, and this circumference determines how far the vehicle moves with each wheel revolution.

There is a video somewhere on youtube where a guy demonstrated this in a parking lot, but I had no luck finding it yesterday. I don't know how the ABS determines tire pressure... couldn't find any details on that either.

 

[rruff]

… a true 37”

It's pretty common for "37s" to be ~36.5". 'Merican units are to nearest inch. They "cheat" on width also, with 12.5" tires measured on 10" rims.

This site has dimensions: https://tiresize.com/tiresizes/37-inch-tires/

 

[rruff]

flatten a tire in a rig with a locked diff on pavement and report back.

If you like you can measure this easily enough, and you don't need a locked diff. Just measure how far 1 rev gets you at near-flat vs full psi.

 

[rruff]

... like this guy did...

 

[p nut]

... like this guy did...

rruff- I’m still not convinced but I’m not arguing that I’m right either. 😁 I’m just really curious to myth bust this and get to the bottom of it.
That video was good but wish they would’ve used a bigger than a 27” tire. I don’t know if there was any meaningful data that could be gathered with such small difference in effective tire size and given the crude way of measurements. Majority of the comments in that video were not favorable, either.

Here is my line of thinking:

We all agree that two wheels of differing circumference (at full pressure) will travel different lengths at one revolution.

Let me give you a scenario:
35 x 12.5 x 15 wheel (at full pressure) has a diameter of…..well, 35”

So the circumference is = 35 * 3.14 = 110” traveled per revolution.

Let’s take the tire completely off the rim. Now we have a wheel that is 15”. So 15” x 3.14 = 47” traveled per revolution.

Now let’s stick the tire back on without putting any air in it. Let’s just say for the sake of the example, it’s a tire with very supple and fragile sidewalls (Goodyear 🤣), and you’re riding right on the rim.

Are you saying by simply mounting the tire (without any air) would net you over twice the distance per revolution? It doesn’t make sense to me.

 

[rruff]

Let’s just say for the sake of the example, it’s a tire with very supple and fragile sidewalls (Goodyear 🤣), and you’re riding right on the rim.

I'll let you do that experiment! But if the tire isn't slipping on the rim or on the ground, then ya... close to the tread length. I think you'll need to put some air in the tire though, to satisfy those criteria.

Yes the test was crude. It would have been better to deflate it more. But he showed that the difference in travel/rev was very tiny... much less than the radius reduction.