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

2449 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.
ok thank you! the full size spare is better than the donut. & since the 2 sizes are so close, I can keep the spare I have in there now, for the next 6? weeks
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nobody wants OEM temp spare donuts. I've got 2 in my basement, Cherokee & Corolla. I only keep them because when I trade in cars, I put them back in the car(s) & I keep the full size wheel & tire because I can sell them or use them in the next car
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I have ADI. it was a no brainer to just buy a wheel & tire
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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
absolutely. handful of years ago, picked up Daughter at college, 200 miles away. got to campus & ripped a tire on a curb. yeah brilliant Dad. put on the full size spare, loaded the dorm room into the Jeep & drove 200 miles home
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OK, next up is another Cherokee this time w/ 17" wheels & I kept my full size spare

looking at the tire comparison charts I think I can still use the full size spare I have. diameter wise it's still a better match then the OEM temp spare, right?

car tires = 225 65 17
full size spare = 235 55 18

comparison chartrs

my concern is being 200 miles away and having to drive the highway back home at 50mph w/ the temp spare, before getting a tire repaired. w/ the full size spare I can drive home at normal highway speeds

this new to me used car, will need a set of tires soon & when I do that, I can get an extra 17" wheel & tire the same size as the rest. but for now, is my full size a better spare option? if not I'll just stick it in the basement until I get Cherokee #3

any thoughts?
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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%.
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
@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!
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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 ;)
2014 I'm told. I get the registration Tuesday
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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 !!
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!
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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... ;)
thank you
it's got the 2.4L
yeah, I'm not throwing money at it right now except for tires soon
I'll try peaking at the battery just cuz I'm curious
gotta check other stuff tho like air filters
the bottom isn't too bad, needs a cleaning then I might brass brush the red parts & brush on some rust restorer here & there
the interior is mint. I need to amorall my all weather mats. I cleaned them but they look old on the new carpet
the tires were 35-37 today. I'm not lowering them
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