JKU Smaller Spare Tyre

[st_za]

Hiyall

I have 285/70/17 tyres which equate to 33" on the nose but the spare tyre is still the stock wheel and 245/75/17 Goodyear 31.7" tyre. The drivetrain is completely stock on my new 2015 Sport petrol JKU.

Is there any risk to any of the components if I run this smaller spare tyre if required? If so, to which components?

Obviously it won't be used in anger and it will be to drive to the nearest tyre shop but considering that Murphy's Law states that it will happen when I am in a far flung area and probably off road, I don't want a flat tyre to turn into a diff/gearbox/transfer case/axle/etc. repair job.

Then again, getting a bigger spare tyre will probably require an aftermarket tyre carrier too (if only in the longer term) and that in turn requires money that I really don't want to spend right now. Or will a 33" spare require a stronger carrier?

Any advice will be appreciated.

Cheers.

[Paul-JK]

Not sure what would happen if you tried to run your spare. With open diffs it shouldn't (I don't think) create any driveline issues. I'd expect the electrics might throw a fit though as the the wheels on that axle will then be spinning at very different speeds. Whether it just lights up the dash but is still drivable would be the question. It might start cutting power trying to compensate for the apparent wheel spin and make the car pretty much un-drivable.

You can fit 33's on stock type carriers. Many have. Mine started rattling badly so I swapped it for a Teraflex door hinge & carrier but many don't have that issue.

For me I would want the peace of mind that my spare was 100% OK to use. Same size and same capability. Otherwise, as you say, Mr Murphy will show his face when you're 100's of km's from nowhere and probably stuck in the middle of a mud bog where your OE spare isn't going to offer much grip and with your JK trying to cut power to the spinning wheel it'll be a long drive home.

[chris0375]

I had 285 75 16 on my rear door for 40,000 km. No issues and plenty of 4wding and rough roads buy the correct size and rotate all 5. Per km driven it won't cost you any more.

[LeighP]

Agree with the comment that it'll probably cause some of the electronics to have a bit of a dummy spit.....the stability control etc.
You should be able to fit a 285/70x17 on the stock carrier....you may need to trim the side plastic anti vibration supports a touch...or maybe not.
If you're worried about the stock hinges, try these heavy duty replacements....



They're made by MORryde and they're sold through most Jeep parts suppliers....greaseable pins on them, too.

[rjvines]

i have 285/70/17 mudhogs on stock rims and use the stock carrier
I was going to fit the terraflex door hinge but was told it was not needed by the person I was going to buy it from

[samft]

given that JK's have no center diff we get the same rotations sent to the front and rear axles.

this means 1 turn per wheel on the fronts (total of 2), and 1 turn per wheel on the back (total of 2) true in corners or in a straight line

the matching tyres will rotate on average once per wheel and will travel an distance of (2*pi*33)/2 or 103.7inches

the diff on the other axle will try and compensate for the miss-matched tyres but the average rolling distance will be less (pi*31.7)+(pi*33)/2 or 101.6 inches

i.e. the axle with matching tyres will try and move 2 inches further than the axle with the smaller tyre (which is bad obviously) this will create wind up in the diffs until the wheels slip (or until failure of the diff, i presume)

this means you can use a non matching spare if you are in 2wd, but not in 4wd

2 inches of slip isn't a huge amount though, and you would probably get away with it on sand where the tyres are slipping anyway.

You will also probably get an electronics error but should be able to drive still, i think traction control would be disabled.

I just made all that up based on my understanding of diffs and the JK transfer case, feel free to shoot me down anybody?

[samft]

I think a 33 would be ok on the stock rear door though, and would probably recommend getting a full size spare