upgrading to 285/6518 tires

No worries .
I would upgrade the 4 tires on the ground .
I would leave the spare the same
I upgraded my tires on my 1997 Vortec 7400 chevy right away to 265/75R16 load range e from the stock 245/75R16LRE stockers .
I did the same on my 2002 duramax .
I used my spare one time in those 17 years .
I credit the tough load range E cords for not allowing easy penetration of road hazards .
Four tires rated at 3415 # each for a pickup rated at 9200 # gvw is no challenge for the upgraded tires .
The additional surface area decreases the load per square inch on the tires .
So they are less likely to succumb to a road hazard .
The spare on my 2002 duramax has never been on the ground .
I practice and believe in upgrading my pickup , TT , and boat trailer tires .
I never have a flat .

Salty Dog wrote:
The rim size is immaterial for spare tire selection - you need to be aware of the outside diameter of the 18" tires and the 17" tires. We had a car with 17" wheels and the spare was 16" and had a higher profile so the outside diameter was equivalent. Check wrecking yards, eBay, etc for cheaper wheels for your spare. Good luck, Mike

Exactly

Stick with 17" and go bigger if thats what you want. Dont ever run an 18" rim with a 17"

The rim size is immaterial for spare tire selection - you need to be aware of the outside diameter of the 18" tires and the 17" tires. We had a car with 17" wheels and the spare was 16" and had a higher profile so the outside diameter was equivalent. Check wrecking yards, eBay, etc for cheaper wheels for your spare. Good luck, Mike

And if you have a limited slip rear end, and put a smaller tire on one side you will destroy the limited slip in just a few miles. Chris

Not a good idea on a axle with diff gears. One inch dia difference will have one axle turning at a much faster rate which creates heat and wear on the side gears. Now if its just a couple of miles then a slow speed won't hurt anything. However it may be 60 miles to the next tire store.

Also anti lock braking can be affected from different diameter tires.

I would have the dealer mount a taller 17" tire on the wheel to match the 18" tires diameter with in reason. If the tire dealer is savy he can answer these type questions.

Be sure the new wheels you purchase are rated for the higher weight also.

Personally, I'd want to be within 1/2" of diameter. If you go more than that, you may run into problems with ABS wheel speed sensors. Also on the 18" tire, h ave you considered 275/70r18? That's a stock tire size for the newest RAMs. So it's a good bit cheaper to buy that size than 285/65r18 for many more brands of tires. the 275/70r18's are only 1/2" taller than the 285/65r18. As for 17" tires, you can look at 285/75r17, but they have dramatically less capacity at only ~3200 lbs per tire versus 3650 lbs per tire with the 18" rim sizes.

On my F-150 when I upgraded to LT tires, it was actually cheaper for me to buy a 34" tire versus a 33" LT tire. I had 20" rims and the 275/65r20's were cheaper than 305/55r20's. The reason was that 275/65r20's are a stock Super Duty tire size, so manufacturers move more of that size tire and it's easier to source at more tire shops. So just more food for thought.