I got 285/65r20 BFGs...need to upsize spare tire?

[Rod Strawderman]

I picked up some 20-inch takeoffs from a High Country with 285/65r20 BFGs. They look great!  

 

I believe the spare is sized to the original 265/70R17 tires.  I believe there is a 3 inch increase in diameter for the new wheels/tires.   Two questions...

 

I assume I need a new spare.  I think I need a 295/75r17 (which is a 34.5 inch tire). Am I right? 

 

And can I fit that tire in the spare tire spot under the bed? 

 

Thank you for your thoughtful responses. 

 

RS

[gemarsh]

I did, only because I bought a 5th matching rim and plan on 5 tire rotations.

[rjgvt]

I would check your spare or the listed one on the tire placard. What size tires/wheels came with your truck?

 

The 285/65R20 tires are 34.6". 

 

The 295/75r17 tires are 34.42"

[Rod Strawderman]

My truck (2022 2500hd LT) came with 265/70r17 and that is the size for the spare under there now.  I worry about the 3 inch diameter difference and the traction control/limited slip rear end (if it has one).  Especially if I have a flat when pulling my camper...  

 

Do I need to worry?  

 

And will a 295/75r17 fit under the bed? 

 

Thanks in advance. 

[Epsilon Plus]

A 37x12.50R18 will fit there (roughly 315/80R17 in metric with 17" rim)

 

Edited March 26, 2025 by Epsilon Plus

[Another JR]

You want an air gap or insulation between the tire and the exhaust heat shield. The shield absorbs the radiated heat from the exhaust pipe, and is cooled by the moving air, keeping the tire from directly receiving the radiated heat. If the shield is in contact with the tire, the tire surface will receive a lot more heat and reach a higher temperature. I don’t know how high, but I assume GM put the heat shield there for a reason. 

[Rod Strawderman]

I will be sure to leave an air gap, but sounds like I will be okay to get that tire into the spare tire spot under the bed. 

 

Thank you. 

 

RS

[Epsilon Plus]

I have a gas truck. That tire could lay up directly against the pipe in that spot and be just fine as could your hand. It's probably for DPF laden diesels.

[Handy]

I have a stock 275/70-18 (33.1") spare with 295/70-18 (34.2") driving tires.  I went through the same thought process and decided to leave the smaller stock tire as a spare.  It is a 1.1" difference, which is a lot less than your difference.  My reasoning was that in 40 years of driving a pickup truck on and off road, I have never had a flat tire.  One reason that I am a fan of the BFG All Terrain T/A tires.  My last truck was 17 years old before it was stolen, and the original spare had never been dropped down. 

If necessary, my plan is to put the spare on the front to avoid a problem with the rear differential.  Reading stories from people who have temporarily driven with a smaller tire on the non-drive axle, it is not a significant problem.

I also wondered about the larger spare fitting, but from that other reply, it looks like it will.  

[Black02Silverado]

I got rid of my 20" and went with the factory ZR2 18" wheels and got an extra one and mounted the same size tire on it for a spare.  I went with 285/70-18 Toyo Open Country ATIII's and found a practically new Toyo M/T tire on FB market place for $25 and used it as my spare.

 

[MG386]

The G80 locking rear differential in these trucks needs 100 or more rpm difference between the two wheels to lock up.  you're not going to see that much difference with a different sized tire.  Think one wheel stationary and the other spinning free.  It takes a lot.

[jake111]

Having different sizes on the back is still not a good idea unless you are only going a very short distance.  While it may not be enough to lock the differential (yikes!!) it's like being in a continuous turn and the differential will be a-spinning.  It's not made to spin like that for very long.

 

If you have different size wheels, put 'em on the front and don't engage 4wd (except to just get unstuck, if necessary).

 

I agree that I haven't had a flat on the truck, so having a different size spare doesn't seem like a problem to me.

[PBNB]

I went to 295/70R18 Falken AT4W's on my Z71 3500 and when I got the tires, I had the same tire put on the spare wheel So all 5 tires are the same.

 

I really don't know how sensitive the diff's are but I really didn't want to go down that road with a space saver spare.

 

The larger spare does fit quite well back there. I think these tires are about 34" outside diameter.