I just bought a set of Michelin defender LTX 275/55r/20 tires for my 2018 XLT. According to Michelin’s website the tire diameter is 31.9”.
The door placard indicates the OEM tire is the 275/65r/18. The typical tire diameter for this size is 32.1”.
Safe to assume the original owner upgraded the tire/ rim combination. The spare is the OEM 18” size.
My question is in the event of a flat tire would using the spare cause problems due to the .2” inches in diameter difference?

 

The spare shouldn't cause any issues if you use it short term as a spare. The rear differential, even if you have the ELD, is open at speeds above 20 mph so that will not cause any issues if the spare is rear mounted. I doubt you would be using 4wd if the spare was front mounted and that differential is open as well.

The difference in wheel speed may prompt issues with the ABS system if it is beyond the allowable tolerance. You will still have brakes, but maybe not all of the ABS functions. If that is an issue, it should resolve itself when you install the regular tire again.

You will get a TPMS error as the spare does not have a sensor. That should go away when the regular tire is reinstalled.

BTW, my truck has 20" wheels but an 18" spare right from the factory.

 

Thank you. I was hoping this was the case and I didn’t need to buy a different spare.

 

I like using www.tiresize.com/calculator for a nice visual.

 

Im curious why you went with an outdated size when Ford went to a 60 aspect ratio on the 20" wheels. Gives you almost the same sidewall as the 18" +/- 1/2"

 

As does mine, which is a little annoying, but I know why Ford did that and I know it will work OK if ever needed. I haven't used a spare tire in many years; I carry a plug kit and compressor, and find plugging screw holes faster and easier than swapping out tires. Then I can stop by Discount Tire and have them patch it from inside (for free) at my leisure.

 

That difference amounts to a whole 3/16 in. Measure your tread and you see how much a tire diameter decreases over it's life just don't for get to multiply by 2. The 18" spare is pretty common as it seems Ford put whatever was available that day in the carrier.

 

Tire rack showed this size for my truck with 20” rims. Costco, where I purchased them, showed the same. I wanted to stay stock if possible.

 

275-60-20 is the latest "stock" size so you wouldn't be deviating from anything since the suspension is exactly the same.
Im sure the combo service writer & risk management coordinator at costco did you a disservice.

 

whatever.