20" tires and an 18" spare on a new ram 2500
 

I recently took delivery of a 2016 RAM 2500 CTD with 20” tires. The four tires mounted on the vehicle are Firestone Transforce AT OWL LT285/60R20.
However, the spare tire is a Firestone Transforce HT BW LT275/70R18.

At my initial service check, when I asked the service man about this difference, I was told that both tires were the same diameter and circumference and that there is no problem operating the vehicle with the spare as provided and that all trucks with 20” wheels/tires are delivered with 18 “ spare tires.

I am not comfortable with running an odd-size tire on the vehicle, even in an emergency/spare use condition. I question the dealer response.

Does anybody else out there with a 2500 RAM running 20” tires/wheels have an 18” spare or are all five tires matched???

If you think about all the cars out there with "compact" spare wheels, people have been using them for many years because the circumference of the tire is the same as the mounted wheels, what you can not do is run the same size rim with a different circumference tire on a 4X4.

The only thing that matters is overall circumference on the tire, not the wheel size. As long as the total height is the same, it wont matter. It might look stupid for a little bit, but its just temporary as it is a spare.

I wouldn't worry about it. Flats in 10-ply tires are extremely rare anyway.