Spare tyre size dilemma...

[BarneyB]

Howdy gurus,

My daily set of rims and tyres are stock STX rims with 265/65/17 A/T tyres.

My second set, for off road & trips are 17x8 steelies with 285/70/17 M/T's.
(I have a fifth rim, but only 4 big tyres at this stage)

The spare under the tub at the moment is a 16x7(?) with...I think... a 255/65/16 highway tyre.
So it's pretty useless at this point.

So, what's the best spare to pair because I care about the air down there and don't need a scare, just wanna compare

Should I...
a) get another 17x7 265/65/70 as the regular spare?

...and/or

b) pick up a decent used single 285/70/17 M/T tyre, fit it to my spare rim, to swap over with the other 4 for trips.

...or...

c) try to carry a hybrid sized spare that splits the difference, and save some extra mucking around a few times per year. But risk drivetrain damage if I had to use the slightly mismatched size for a long distance?

...or....

d) other...

 

[KevinE]

Seeing as the spare is only there to get you to where you can get the flat tyre repaired or replaced, it depends on where you trip?

If you're going somewhere really remote, where there isn't anywhere to get your flat fixed/replaced, I'd match the spare to the tyres you're using on your trip.

But for most trips, including outback, there's pretty much always somewhere not too far away to get a tyre repaired, or buy a new one.

Genuine question though, I'm wondering why you use M/T's for trips & leave your AT's at home?

 

[BarneyB]

Seeing as the spare is only there to get you to where you can get the flat tyre repaired or replaced, it depends on where you trip?

If you're going somewhere really remote, where there isn't anywhere to get your flat fixed/replaced, I'd match the spare to the tyres you're using on your trip.

But for most trips, including outback, there's pretty much always somewhere not too far away to get a tyre repaired, or buy a new one.

Genuine question though, I'm wondering why you use M/T's for trips & leave your AT's at home?

Most off road trips are somewhere in Tassie, so in theory nothing is very far from a repair shop. But you'd be amazed how "remote" it can get down here sometimes. It might only be 100kms to a workshop, but it'll be over 2 mountain ranges on a goat track that takes hours & hours y'know.
Next year we're planning to go up the east coast to a few places, including Fraser Island. So that will need more preparation.
And I'd like to do the Vic high country soon too.

I'm leaning more towards 2 different correct spares, because the rear diff is LSD and there's a Lokka centre going into the front this winter.
A quick look at the tyre size calculator shows that a 265/70/17 splits the difference closest to the middle.
But I really don't know if/how long the driveline would tolerate the difference until I broke something to find out

 

[KevinE]

I guess that answers my Q about M/T's, you're in Tassie

We do a bit of outback touring, but not often very remote. We've driven the usual stuff like the Gibb River Road, the Bungle Bungles, Parry’s Lagoon, the Old Jim Jim Road, Rainbow Valley, the Oodnadatta Track a few times, the Mereenie Loop, Palm Valley, Chambers Pillar, Mt. Dare a couple of times, Dalhousie, the Old Ghan Heritage Track, The Painted Desert, Lambert’s Centre, Santa Teresa, Cameron Corner a few times, Innamincka via Epsilon & via Merty Merty, the Dig Tree, the Warri Gate, Noccundra, the Dowling track, Hungerford to Eulo, Betoota a couple of times, Birdsville a couple of times, (drove it up Big Red lol), the Birdsville Track, the Strzelecki Track twice before they sealed it, Roper Bar, the Nathan River Road, Lorella Springs, the Savannah Way through Hells Gate, the Gawler Ranges a couple of times, the Flinders Ranges (Many times), Arkaroola twice, White Cliffs to Wanaaring via Peery lake, the Hamilton Gate, Haddon Corner, Bedourie, Boulia, Donohue/Plenty Hwy, the Binns track, N’dala gorge, Altunga, East & West MacDonnell Ranges several times, Nuccaleena, Morgan/Yunta Mail Run, the Borefield Rd, Mt. Ive to Kingoonya via Lake Gairdner, Tarcoola, the tracks north of Andamooka, Lake Torrens, Italowie & Weetootla, Grindall’s Hut, the Balcanoona/Moolawatana Road, Chamber’s Gorge, Lake Frome.

We got home from a trip last Thursday where we headed up to Cameron Corner again & then out to Merty Merty. Drove on up to Innamincka & it had rained north of there & all the roads were closed We'd been heading to Birdsville. So we had to improvise. We back tracked down to Tibooburra via Cameron Corner, had a day at Milparinka & then drove the Cut Line to Wanaaring. Went & had a look at Fords Bridge, then Gundabooka NP, then did the Darling River Run on the way home (hadn't done it before, so why not lol)

The only times I've felt truly remote in our travels was out on the Nullarbor regional Reserve & doing the Old Andado track.

My point in telling you all of this? I have 265/75/R16's on the car & the spare is 265/70/R16. If I get a flat in the rear axle (LSD), I'll swap a tyre off the front onto the rear flat & put the spare on the front.

A locker won't matter if it's not engaged.

Food for thought

 

[BarneyB]

I guess that answers my Q about M/T's, you're in Tassie

We do a bit of outback touring, but not often very remote. We've driven the usual stuff like the Gibb River Road, the Bungle Bungles, Parry’s Lagoon, the Old Jim Jim Road, Rainbow Valley, the Oodnadatta Track a few times, the Mereenie Loop, Palm Valley, Chambers Pillar, Mt. Dare a couple of times, Dalhousie, the Old Ghan Heritage Track, The Painted Desert, Lambert’s Centre, Santa Teresa, Cameron Corner a few times, Innamincka via Epsilon & via Merty Merty, the Dig Tree, the Warri Gate, Noccundra, the Dowling track, Hungerford to Eulo, Betoota a couple of times, Birdsville a couple of times, (drove it up Big Red lol), the Birdsville Track, the Strzelecki Track twice before they sealed it, Roper Bar, the Nathan River Road, Lorella Springs, the Savannah Way through Hells Gate, the Gawler Ranges a couple of times, the Flinders Ranges (Many times), Arkaroola twice, White Cliffs to Wanaaring via Peery lake, the Hamilton Gate, Haddon Corner, Bedourie, Boulia, Donohue/Plenty Hwy, the Binns track, N’dala gorge, Altunga, East & West MacDonnell Ranges several times, Nuccaleena, Morgan/Yunta Mail Run, the Borefield Rd, Mt. Ive to Kingoonya via Lake Gairdner, Tarcoola, the tracks north of Andamooka, Lake Torrens, Italowie & Weetootla, Grindall’s Hut, the Balcanoona/Moolawatana Road, Chamber’s Gorge, Lake Frome.

We got home from a trip last Thursday where we headed up to Cameron Corner again & then out to Merty Merty. Drove on up to Innamincka & it had rained north of there & all the roads were closed We'd been heading to Birdsville. So we had to improvise. We back tracked down to Tibooburra via Cameron Corner, had a day at Milparinka & then drove the Cut Line to Wanaaring. Went & had a look at Fords Bridge, then Gundabooka NP, then did the Darling River Run on the way home (hadn't done it before, so why not lol)

The only times I've felt truly remote in our travels was out on the Nullarbor regional Reserve & doing the Old Andado track.

My point in telling you all of this? I have 265/75/R16's on the car & the spare is 265/70/R16. If I get a flat in the rear axle (LSD), I'll swap a tyre off the front onto the rear flat & put the spare on the front.

A locker won't matter if it's not engaged.

Food for thought

Lokka, not locker.
No user control over engagement behaviour. (apart from throttle input). Would be unlikely to tolerate different diameter tyres, but I should prolly ask 4wd Systems.

Thanks for the long list of places you've been. I'll never get those 2 mins back.

Not sure what food I'm supposed to be thinking about, but I love burgers and curries

 

[Old.Tony]

I agree with Kevin, if you're going to have an odd sized wheel in the mix, it's best to put that on the front, LSDs can and will break. I don't know why a centre locker is even needed, the transfer case is a direct engagement - a front lokka (auto) will only engage if you have 4HI or 4LO active because it depends on the input shaft being turned.

Why is a central lokka being installed?

 

[BarneyB]

I agree with Kevin, if you're going to have an odd sized wheel in the mix, it's best to put that on the front, LSDs can and will break. I don't know why a centre locker is even needed, the transfer case is a direct engagement - a front lokka (auto) will only engage if you have 4HI or 4LO active because it depends on the input shaft being turned.

Why is a central lokka being installed?

No centre diff lock fitted, or plans to...(??)
Not sure how that's possible?

As you said, front auto Lokka should be passive in 2H.
(Although, I've read they can sometimes get a little "catchy" in some vehicles, for lack of a better word, during normal 4x2 driving. Nothing bad, just how they are. I wonder if different axle speeds, from different tyre sizes, might induce unwanted partial engagement?)

Anyway, my concern is different diameter tyres when front Lokka is active (4H & 4L). This may not be a scenario the Lokka was designed for? But it might be necessary when if/when I need to finish the track to get home. i.e. The flat tyre might be half way along a 4L level track...

TBH I'm leaning towards 2 different spares - one for each set of wheels.

I posed the question because I thought maybe there's a "cheater" size that would cover both sets... but the more I think about it, the less I like it.

 

[Old.Tony]

The issue is the circumference of the tyre. If there's different circumferences, the wheels need to rotate different amounts to cover the same distance - and a fully locked driveline won't allow that, so one wheel will have to slide.

On a loose surface that's not too bad. Give the wheels a little grip and you'll feel it binding.

 

[D40-12-Xtra-Newc]

Tyre Circumference: Just to be technically correct, the important measurement is "Rolling Circumference" - can be different from static, or off-vehicle measure, especially if 'airing down'.

 

[Rhino1174]

Being a Tasmanian I get the remote feeling especially Sunday morning west coast broken not a hope of making it anywhere to get parts or tyres feeling so I went the route of matching my spare to my off-road tyres just for peace of mind i don’t want to break anything else because I was lazy

 

[BarneyB]

Tyre Circumference: Just to be technically correct, the important measurement is "Rolling Circumference" - can be different from static, or off-vehicle measure, especially if 'airing down'.

Yeah very true.
In theory, if you had a spare that was very close to the RC of the other three tyres, you could under or over inflate it to manipulate the rolling circumference. But nearly impossible to measure accurately knee deep in shit at 10 o'clock at night in the bush

 

[BarneyB]

Being a Tasmanian I get the remote feeling especially Sunday morning west coast broken not a hope of making it anywhere to get parts or tyres feeling so I went the route of matching my spare to my off-road tyres just for peace of mind i don’t want to break anything else because I was lazy

Yeah I'm sure the mainlanders probably laugh when we say "remote" or "stranded" or "a long way from home"
The distances are not huge in Tas, but the West Coast can be very quiet and lonely when you need help.

 

[Rhino1174]

Yeah I'm sure the mainlanders probably laugh when we say "remote" or "stranded" or "a long way from home"
The distances are not huge in Tas, but the West Coast can be very quiet and lonely when you need help.

Well I know I certainly wouldn’t want to walk or leave my vehicle on some of these tracks it would either sink and never be found again or be stripped bare when I got back to it so it’s remote to us