Things to know about 33" tires

Good morning everyone. Learning as I go, Ive never had a truck with 33" tires and have a few questions.

Recently purchased my truck and it came with 18x9 Fuel Coupler wheels with LT265/70R18 Nitto Ridge grappler tires. Id like to know which wheels I have, but don't know the offset. Options are 1, -12, or 19:
https://www.fueloffroad.com/fuel-1p...O6pUtTg-tNPyqrYhNFH6x-afOA-0vzdSJQIbZGTRlKxdm

Any tips on figuring this out?

The truck currently has the original spare 265/70R16. I've been told I cant run that as a spare on the rear because it will trash my rear differential. True?

The best would be to get a similar sized tire and rim as the rest of the truck, right?

Thank you all!

 

Have you tried taking a rim and tire off the truck to see if it is stamped or engraved with offset and size?

 

I havent taken it off yet. Ive crawled underneath and looked at the inside of the rim. Other than weight specs, its stamped with 18x9.

 

Easy way to determine offset is backspace...on an 18X9 wheel:

1 = 5.04"
19 = 5.75"
-12 = 4.53"