20” tires and an 18” spare on a new ram 2500 ctd

[Jwskj]

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???

 

[Badunit]

Your service guy is correct. My 2014 came that way, too.

LT285/60R20 = 33.5" diameter, nominally
LT275/70R18 = 33.2" diameter, nominally

You'll get more variation that that from air pressure and wear. The two sizes are functionally identical.

 

[kthaxton]

Agreed and.......forget about the wheel size, you need to be concerned with the tire size (diameter).

 

[Alphonse]

You already have your answer but to put some specifics on it, I have a 2016 2500 and it came with the following:

Vehicle tires: LT285/60R20 Load Range E Service 125/122R Firestone TransForce AT
Spare tire: LT275/70R18 Firestone HT
Vehicle tire load rating 3640#/tire at 80 psig; 56 lb., 33.4” diameter; Spare - 3640# at 80 psig, 50 lb., 33.2” diameter

The spare is 0.2" smaller in diameter. That means 0.63" less in circumference (negligible).

We're good to go with a 18" spare on a steel rim.

 

[Jwskj]

I understand the diameter thing, and that the is most important, and that both tires have effectively the same dynamic diameter, but then that prompts the logical question 'Why wouldn't RAM simply provide a spare tire identical to what is mounted rather than providing an "almost the same" wannabe?
It isn't a question about fitting in the spare tire cavity because both tires are effectively the same diameter. With the current cost of these new trucks, it is unimaginable to me that someone at RAM/Chrysler would make a decision to do that in the interest of cost effectiveness, if that was actually the intent.

 

[Alphonse]

I think it because an 18" tire on a stamped steel rim cost them and us less.

 

[Newsa]

Buy a 20 inch on Ebay as your spare

 

[Badunit]

No need for an identical tire. The wheel would cost more. The tire would cost more. There is no financial or useful advantage to the owner. The 18" steel wheel spare might stand out like a sore thumb when you use it but so would a 20" steel one.

 

[Newsa]

I put my steel spare (no chrome overlay) into rotation with my other tires. It looked bad and set off my TPMS light constantly. But it was identical to the others in all other ways (except chrome & TPMS sensor)