Forum Discussion

leggy's avatar
leggy
Explorer
Oct 07, 2017

Different radius tires on truck

Getting ready to roll this afternoon, doing my last check of tire pressure I notice a screw in the tread of one of my tires. Off to the local garage and get a plug put in. Back home, hook up and finally on the road. Ten miles down the road my truck brain says low tire pressure. Ugh, guess the plug is leaking. Pull over at a local firehouse, nice large lot, get out to look at troubled tire... perfectly fine! Rear passenger on the other hand, going flat quick.
Pull behind firehouse so I'm not blocking the doors and change with spare. Umm wait, it is 18" not 20 like the rest. I bought the truck used and never thought to check that. So, guess we'll have different radius tires at least until tomorrow. Oh, wait only 43lbs in that one. Good thing I have my pancake compressor with me and fully charged up with air. Great, now I have 59lbs. Hmm, there's an outlet on the wall of the firehouse and it works. Good to go with pressure.

So, while going down the road my mind got thinking about what bad things I might be doing to the rear differential. It handled fine but going to try and get a new 20 tomorrow so I'm not thinking about it all the way home.
Tow Vehicles
  • ScottG's avatar
    ScottG
    Nomad
    Oct 07, 2017
    The spider gears will turn constantly, which they aren't designed to do, with different diameter tires. It's best not to do sustained high speeds that way and get the correct tire or stuff will wear.
    It's also possible that the different wheel sizes dont result in very different OD's. It all depends on the aspect ratio's of the tires invovled.
  • camp-n-family's avatar
    camp-n-family
    Explorer
    Oct 07, 2017
    It's the tire diameter that matters. A 20" rim can have the same diameter as an 18" because the height of the sidewall is different. For example my stock size is 285/60r20 and my spare is 285/70r18. They are within 0.3" difference and are fine to use at the same time for a spare. Most trucks with 20" stock rims will have an 18" spare.
  • camp-n-family's avatar
    camp-n-family
    Explorer
    Oct 07, 2017
    Check this Tire size comparison to see if the sizes are compatible. If they are within 1 percent they are fine to use together. If they are mismatched, put the odd tire on the front axle and don't use 4 wheel drive.
  • ktmrfs's avatar
    ktmrfs
    Explorer II
    Oct 07, 2017
    camp-n-family wrote:
    It's the tire diameter that matters. A 20" rim can have the same diameter as an 18" because the height of the sidewall is different. For example my stock size is 285/60r20 and my spare is 285/70r18. They are within 0.3" difference and are fine to use at the same time for a spare. Most trucks with 20" stock rims will have an 18" spare.


    likewise on my truck. I bought it brand new, came with 20 inch tires and the spare is an 18", regarless if the other tires are 17, 18 or 20 inch from the factory. In fact all three tire sizes have virtually the same diameter. the amount the other tires are worn compared to the spare is as big or bigger difference in diameter.
  • hedge's avatar
    hedge
    Explorer
    Oct 07, 2017
    AFAIK it is quite common for the manufacturer to supply a spare with a different size rim so I don't think the previous owner pulled a fast one on you.

    As said above it's the overall size that matters, not the size of the rim. Their may be some difference but I don't think the spare is meant for long-term use.
  • Durb's avatar
    Durb
    Explorer
    Oct 07, 2017
    It might also depend on what kind of electronic nannies you have on your truck. I had a tire go bad on an AWD SUV and I had to replace all 4 tires because of different rolling diameters and this is just from tire wear. The computers read the RPM of each wheel and will apply the brakes to a wheel if it considers it to be slipping. Not at all good for the vehicle long term. If your truck is "new" and has whiz bang traction control then I would check the owners manual very thoroughly. Not saying this is the case but something to think about.
  • Grit_dog's avatar
    Grit_dog
    Navigator
    Oct 07, 2017
    First see what the difference in tire dia is. Rim size has nothing to do with it.
  • Bedlam's avatar
    Bedlam
    Moderator
    Oct 07, 2017
    As posted, tire diameter has little to do with rim diameter. I used a 275/70R18 spare for my 245/70R19.5 tires - Both were the same diameter tire but different width on different diameter rims.
  • Groover's avatar
    Groover
    Explorer II
    Oct 08, 2017
    hedge wrote:
    AFAIK it is quite common for the manufacturer to supply a spare with a different size rim so I don't think the previous owner pulled a fast one on you.

    As said above it's the overall size that matters, not the size of the rim. Their may be some difference but I don't think the spare is meant for long-term use.


    While this is certainly true I cannot understand why manufacturers don't offer a matching spare tire option including tire pressure monitor. I would have gladly paid for it. Seriously, how much trouble could it possibly be to grab all 5 tires from the same bin?
  • mich800's avatar
    mich800
    Explorer
    Oct 08, 2017
    Groover wrote:
    hedge wrote:
    AFAIK it is quite common for the manufacturer to supply a spare with a different size rim so I don't think the previous owner pulled a fast one on you.

    As said above it's the overall size that matters, not the size of the rim. Their may be some difference but I don't think the spare is meant for long-term use.


    While this is certainly true I cannot understand why manufacturers don't offer a matching spare tire option including tire pressure monitor. I would have gladly paid for it. Seriously, how much trouble could it possibly be to grab all 5 tires from the same bin?


    Cost is a big one. Plus these are not full time 4x4 so treat it as a temporary solution like other temporary spares and get the damaged tire replaced ASAP.

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