20"tires with an 18" spare

[daryl]

Hi everyone. I bought my RAM used back in Nov. It has 20" rims. No biggie. I crawled underneath to change out water separator/fuel filter and noticed I it has an 18" spare. Would I be able to use spare as long as I don't put it in 4x4 and turn off traction control? I'm looking at possibly buying new 18" rims and tires. It's gonna need new tires soon anyway.

 

[AH64ID]

What are the tire sizes? That’s more important that wheel size.

 

[daryl]

275/70R18 spare
285/60R20 tires on truck

 

[lpennock]

They are close enough. 18" is 33.2" tall and 20" is 33.5". The 18" will turn 5 more revolutions over 1 mile. About like making a sweeping turn.

 

[daryl]

They are close enough. 18" is 33.2" tall and 20" is 33.5". The 18" will turn 5 more revolutions over 1 mile. About like making a sweeping turn.

Thanks. I wasn't sure and I didn't want to fudge up anything on the truck.

 

[wyosteve]

That is likely the factory tire/wheel combo for the spare. My 2015 which I bought new had factory 20's with an 18" wheel for the spare. I found it strange, but as mentioned, the tire diameter was the same which is the important part.

 

[mfurrh55]

Look inside door post it will tell you what size tires are and it will tell you what size spare; my wife's Ford has 20s and an 18 spare and that's exactly what it says on door post

 

[mfurrh55]

My wife's Ford came with 20s and spare says 18, I looked on door post and it says 20s and 18 spare, I was worried to start with, but its inside diameter, outside 20s and 18 are same on hers

 

[daryl]

I looked on the door post to see what tires came on truck and it was the 20". I'm going to look again to see if it has spare listed. 20" to 18" seems like a big jump but I never thought of tire sizes. I just wanted to be sure I wasn't gonna mess things up on the drive train. This just saved me some money. Now I can just get new tires. Thank you for the replies and all the help.

 

[lpennock]

I looked on the door post to see what tires came on truck and it was the 20". I'm going to look again to see if it has spare listed. 20" to 18" seems like a big jump but I never thought of tire sizes. I just wanted to be sure I wasn't gonna mess things up on the drive train. This just saved me some money. Now I can just get new tires. Thank you for the replies and all the help.

If you decide to change tire size you will also need a spare that is the same height. Alot of people go to a 35" class tire, so you can use a 295/70R18(34.1) or a 285/75R18(34.7) or a 35/12.50R18 depending on the actual size of the 20" tire you use.

 

[nukegm]

i would think that the different sized tires , if put on the rear ( with limited slip ) would at the least irritate the limited slip differential? just throwing that thought out there. cheers!

 

[mfurrh55]

i would think that the different sized tires , if put on the rear ( with limited slip ) would at the least irritate the limited slip differential? just throwing that thought out there. cheers!

No; outside diameter is same, only difference in wifes ford is inside diameter

 

[lpennock]

i would think that the different sized tires , if put on the rear ( with limited slip ) would at the least irritate the limited slip differential? just throwing that thought out there. cheers!

In the case of the clutch pack limited slips running a significant difference in tire size would wear burn out the clutches. These trucks run a Torsen style Limited slip which doesn't have that issue. The Torsen style continuously bias the torque and don't wear just because one wheel is turning at a slightly different speed.

In the case of this thread, the outside diameter of the tires is so close it wouldn't even wear out the clutch type unless it was run that way for many thousands of miles. I is about the same as having a brand new tire and a almost completely worn out tire on the same axle.

Bottom line not an issue in this case.

 

[nukegm]

In the case of the clutch pack limited slips running a significant difference in tire size would wear burn out the clutches. These trucks run a Torsen style Limited slip which doesn't have that issue. The Torsen style continuously bias the torque and don't wear just because one wheel is turning at a slightly different speed.

In the case of this thread, the outside diameter of the tires is so close it wouldn't even wear out the clutch type unless it was run that way for many thousands of miles. I is about the same as having a brand new tire and a almost completely worn out tire on the same axle.

Bottom line not an issue in this case.

excellent explanation. thank you. im 50 and not too old to always learn something. cheers sir!

 

[SnoKing]

If you decide to change tire size you will also need a spare that is the same height. Alot of people go to a 35" class tire, so you can use a 295/70R18(34.1) or a 285/75R18(34.7) or a 35/12.50R18 depending on the actual size of the 20" tire you use.

Is this clearance for a larger spare tire under the truck?

 

[daryl]

I was worried about the different tire sizes messing up the rear. I would only use the spare until I could get tire repaired or replaced.
Thanks Ipennock for the explanation of the Torsen style Limited slip.

 

[lpennock]

Is this clearance for a larger spare tire under the truck?

I can't say for sure on 4th gen. Was enough room on a 3rd gen.

 

[SnoKing]

I can't say for sure on 4th gen. Was enough room on a 3rd gen.

Our second gen with tight with the optional 265/75R16's.