Tire sizing.

ferniesnow

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Hey guys, I have an 18” spare tire but run 20” on the truck. Tire size is 275/65R20 on the truck and the spare is 275/65R18.
There is 1 and a half inches of clearance on both sides of the spare so if a 20” spare is required it would fit.
Ideally, these should be the same size (both should be 20”) but is this a big problem for short distances and lower speeds?
What are the thoughts of the S&M experts?
 

mikeydoo

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0.5% difference in overall diameter. the 18 inch is the bigger of the 2. good to go no problem to use as a spare on the hiway.
 

sirkdev

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I have always been pretty weary of this and get the correct size spare myself, I'm sure it won't hurt for short distances but could be an expensive lesson.
 

Jorg Jorgenson

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Just make sure you don’t put the spare on your rear end and don’t run 4x4 if it’s on the front of the truck. If you’re that worried bout it (I likely would be but im anal when it comes to that)

My neighbour had a rear tire that kept losing air in it and he actually wrecked one side of his TC?? (Nissan frontier) as the wheels were not spinning at the same rate due to the changed circumference of the tire with low air pressure. so be aware!


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JMCX

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Even the rear should be fine as long as you don't use the rear diff-lock on your Ford
 

MP Kid

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Hey guys, I have an 18” spare tire but run 20” on the truck. Tire size is 275/65R20 on the truck and the spare is 275/65R18.
There is 1 and a half inches of clearance on both sides of the spare so if a 20” spare is required it would fit.
Ideally, these should be the same size (both should be 20”) but is this a big problem for short distances and lower speeds?
What are the thoughts of the S&M experts?
Are you certain on those sizes...? Those are the same sidewall dimensions, but on different rims, which equates to significant overall size difference. 6% circumference difference...!!!

If your spare was in fact 275/75r18, that would be much closer (not sure if that size even exists).

I wouldn't get misdirected about the rim size, it's the overall tire size that's important...

I use this site often...
 

ferniesnow

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The tire sizes are correct. When I in-put the sizes in the calculator, the only difference is the diameter of the tire. The 18” tire has a diameter of 100” and the 20’ tire has a diameter of 107”.
 

MK4TDI

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Are you certain on those sizes...? Those are the same sidewall dimensions, but on different rims, which equates to significant overall size difference. 6% circumference difference...!!!

If your spare was in fact 275/75r18, that would be much closer (not sure if that size even exists).

I wouldn't get misdirected about the rim size, it's the overall tire size that's important...

Like MP said, the spare should be a 75 side wall, basic rule of thumb is 5mm for every inch of rim size.

275/55/20 is basically the same as 275/65/18 circumfrence wise.
 

Cableguy

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tape measure the outside rolling size of spare tire compared to the 20s tire
 

acesup800

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One is a 32" tire and one is a 34" tire. That is 6% difference. If the spare is put on the rear, you are going to damage a limited slip or diff lock axle. You will be fine if its an open diff. I thought all new 4x4 trucks have at least a limited slip. If so, the only option is to always put the spare on the front as suggested above and move the front to the back if you get a rear flat. Not sure on the front diff if you need 4x4. I would get a different spare to avoid the hassle.
 

Lightningmike

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lilduke

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with a open diff you'll wear out the spider gears. With a good full time locker you'd just wreck the tires or wont even beable to drive it lol.

Selectable locker is a open diff most the time and you'll wear out the spider gears.
 
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LUCKY 7

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Doug. Check out car-part.com if you plan on buying a tire and rim. You might be able to find one close to your area and save some $$$$$
 
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