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tire math experts - I shud change my spare, right?

2450 Views 34 Replies 11 Participants Last post by  rumrunner
on the verge of putting my winter tires on, but here's the situation

last time I got new tires (April 2021) I changed tire size & bought 5, so my spare would match
... so if I put my winter tires on (which are the OEM size) can I leave that new spare mounted in the cargo area? would it be ok to use in the rear, if I get a flat? or shud I not risk it & put the OEM sized spare on?

both spare tires are new full size tires
the OEM spare was only used once for a cpl hundred miles
the winter tires have a few seasons on them

the OEM size is 225 60 18
tires on the car now are 235 55 18

so can I use winter tires that are 225 60 18 but keep my spare that is 235 55 18?

I guess I know the answer: "yes change all 5 tires" because, why not?
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Well... to put things in perspective :

Would you use the Cherokee donut spare if you had one and needed a spare ?

Donut spare : 165/80/17 = 27.4''
OEM : 225/60/18 = 28.6''
Other : 235/55/18 = 28.2''

4.2% difference between OEM and donut.
1.4% difference between OEM and other.

Math looks good to me.
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I was told at a tire shop when I asked this, that subarus get mad if you go more than 4/32nds different in size between any tires. Which would be 6.25%, so perhaps the 5% thing is appropriate. So for example, on my jeep I have been too lazy to replace my summer tire in the spare area with my OEM full size spare. So if I got a flat tire with my winter tire 235/65/17 I would be replacing with a 265/65/17, which is probably too much of a difference. But if I instead replaced with my 245/65/17 it should be ok. This is good news to me since I have been trying to sell that stupid OEM spare for years. Now I can just keep it as a spare for the winter.
Huh ? How does a 4/32nds difference translate into 6.25% ?

4/32nds difference (this is radius, not diameter) is 1/8'' or 3mm. So X2 for diameter = 1/4'' or 6mm.
Now on a 225/60/18 tire which I took for the math : 28.6'' is spec. Go up 1/2'' (now this is double your 4/32nds) and you're at 29.1'', right ? Well that is a 1.7% difference. So divide by 2 and you're under 1% difference.

Say your guy misspoke and meant the difference between a new tire and one down to 4/32nds : new is usually 11/32nds (some at 12, others at 10). So the difference there would be 7/32nds between new and worn down to 4/32nds.
Ok my head hurts, but a 7/32nds difference = 14/32nds in diameter, so just under 1/2''. Remember the math above with a 1/2'' diameter difference on a 225/60/18 ? 1.7%... So I dunno where they got 6.25%...

If a Subi can't handle a 0.85 or even a 1.7% tire size difference, I'd go shopping for another car ;)
I always thought that donut spare had a speed limitation on it as well. I went with a full size spare so I can keep on truckin' at highway speed to finish whatever highway trip I'm in the middle of. Is there a flaw in my logic?
Correct, I believe they are rated for max. 50mph. They can go faster, but 50 is there for safety reasons that also include less grip, smaller diameter so higher revs on that wheel and I'm sure a safeguard knowing most people don't care for their spare (= grossly inderinflated).
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THE solution: get factory tow whether you tow or not.
Read your Owner's Manual and/or the tire sidewall.
Tow group, or AD2, which gets full-size spare also.
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Well... 8.6mm diameter difference is like a 5/32nds thread depth difference (radius measurement), close to the difference you'd have between a new tire and one at 6/32nds thread depth (new tire being at 11/32nds). It's not a significant difference. Diffference with a donut spare (165/80/17) is more like 4%.
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thank you
so it's better than the temp spare

& it's ok that it's an 18" wheel? the bounce dynamics might be off? but still better than the temp spare
Total diameter is the main concern, wheel diamater, especially a mere 1" difference, is more an aesthetic thing.If you had a 22" wheel with a very low profile tire... then you would likely feel it on the ride .
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@rumrunner : let's get a little crazy for a second 😁

Was thinking about total diameter vs tire air pressure, and found a thread on a Forester forum where this guy did an experirment, measuring height while inflating/deflating the tires.

Anecdotally, he found that a 2 psi difference = 1/32nd change in radius (2/32nd in diameter).

So let's say you keep the spare at 40 psi, you're 4 psi over normal, and roughly 2/32nds bigger (radius). So down from 5/32nds difference at equal pressure (for your spare) to 3/32nds with 4 more psi in the spare. Things are looking up, lol.

And now don't forget you have to deal with 36 psi tires vs your Limited's 33 psi tires. ;)

hehe fun!
lets not mix in what the current tires measure for tread, because they aren't new & the rears are more worn that the fronts (must have been rotated recently) but they are dry & will be replaced soon & I'll get 5
actually the sport doorjamb also says 33psi front & rear!. they were overinflated to 40 when I drove it off the lot & so I brought them down to 34. nice to have 4 working TPMS sensors again!
Having wear on the 225/65s is actually good for spare matching right now, haha.

33 on the 17'' tires ? What model year is the Sport ? I didn't get the memo on that combo ;)
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2014 I'm told. I get the registration Tuesday
View attachment 223165
Ok yeah, as best I can recall, 36 psi came around for 2015+ (some trims with 17''s). 2014 was all 33 psi. Such low mileage for a 2014, wow !!
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oh ok interesting
hehe I know right? it drives great, shifts great. zero indication of all the shifting complaints ppl had. heck it's shifts better than my previous 2014 ever did. I do wish the engine was cleaner & that the undercarriage was newer looking. but then, I'm greedy like that ;-)
it's got the little access doors for the headlight bulbs & I know how to replace those. don't know if it's the original battery, I hope not. it's all covered w/ a blanket so can't readily see any #s. when I went to adjust the time on the clock, the year was at 2015, whatever that means!
Not sure you mentionned it (I probably missed it), but which engine in the Sport ?

Yeah the clock setting : probably a sleeper car lol. Most probable is that battery got badly discharged/died at one point and clock got a reset.

As for tire pressures : as far as I know, the jump to 36 psi in 2015 (for most 17'' tires) happened out of the blue, probably for mpg improvement. You can safely drive yours at 36 all around, if you so chose.

Battery : yep that cover is a PITA. You have 4 options I guess :
1) Remove cover and check (pain...)
2) Use scissors and open up a corner of the top cover, say near the negative post, to see part of the battery. OEM battery is easy to ID (I have pictures of my OEM battery you could look at). Don't do this near Positive post, to avoid possible shorting out with nearby metal). Then use Gorilla tape to re-seal the cover (hoping Gorilla tape sticks to that material... uhm...).
3) Use a battery tester to check status (like the Ancel BA101).
4) Let it live until it dies... ;)
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