Tech Q on Spare Size / Diff Burn Up

[Overlander]

Debating strategy on running a full size spare (35x12.5) vs keeping factory spare (33). I have OBA and have the skills to plug in the field, however a catastrophic sidewall cut would necessitate using a spare.

If using the 33 mixed in with 3 other 35s I'd plan on putting it up front regardless of where the flat was. A potential PITA, though in a decade of trail running I've yet to have a failure.

My Q is: even in 2wd the front diff is still getting wear with one 33 and one 35, right? Just because it's not driving the front with power, the difference ion circumference is still spinning the spiders. Say I have to run this scenario for 200 miles on the highway to get to a place where the 35 can be replaced how much wear/damage will occur? Unloaded the wear should be reduced but it seems like a lot of spinning for those little gears.

Trying to balance the risk vs the additional cost and space requirements for a matching spare.

Thoughts?

 

[Travelin Ram]

Not an issue on the front when you’re in 2WD. The current generation PW uses a center axle disconnect between the front diff and the RF wheel to separate the axle shaft (when in 2WD).

I run with the OEM 33” spare vs 35’s on the truck myself.

 

[SeppW]

You could probably get away with it for a short distance. If you have OBA, why not go ahead and get a 35 spare? Granted, it'd have to be aired way down and finessed up into the spare location. I have OBA and run 35s, including the spare.

AEV has a bed tire rack for the Ram 2500, but it shows out of stock and it will not fit Rambox beds, but according to AEV, they are developing one that will.

 

[Trailmaker]

Not an issue on the front when you’re in 2WD. The current generation PW uses a center axle disconnect between the front diff and the RF wheel to separate the axle shaft (when in 2WD).

I run with the OEM 33” spare vs 35’s on the truck myself.

I 2nd that. I run 35s and OEM spare. Did this on my Jeep as well. Moved it to front if I needed it.

 

[crazy jerry]

You could probably get away with it for a short distance. .

actually you could get away with it for a million miles. and if you stopped and engaged 4x4 (so the shafts could engage from the speed difference in tire diameters), you could continue driving with no worry because its a open differential. just dont engage the locker. as long as the differentials are open, you could have four different size tires at each corner and it wouldnt matter. of course it wouldnt handle very well but thats a seperate matter

 

[Travelin Ram]

Helical LSD in the rear, different sizes on that axle no bueno. Not on pavement anyway.

 

[Breaker Alex]

IMO just find a used 315 or 35 and you'll be good for the life of the truck.

I fitted a used fully aired 315/70r17 BF tko2 easily ( since they run so small )

But I'm on 37s so will have to put in front anyway

 

[62Blazer]

Absolutely no issues with doing this on the front as already mentioned. I wouldn't be concerned about running 4wd either with mismatched front tire sizes assuming you were in a low traction situation that actually needed 4wd, or at least a dirt or gravel road.

If you did it on the rear it's not like the diff is going to explode as soon as you start driving. You might get some weird handling as the limited slip diff will constantly think one tire is spinning faster than the other and try to transfer power, but again don't think it would be anything major when you are talking a 33 vs 35 tire.....not saying you should do this and drive for 200 miles down the freeway but if you were on a trail and needed to do a quick tire change and drive out to the road it should be okay. Just like the comment above if you are on a dirt, gravel, or otherwise low traction surface I don't think it would be a huge ordeal. You can also fully inflate the 33 and run the 35 at a lower pressure to get the sizes closer to each other.