temporary spare tire can I replace it with a matching full-size tire like the rest of my four tires?

You can replace it with a full-sized spare, but you need the correct rim size for the tire you are using not the one the temporary spare is on. They are generally too narrow for the standard tire used. A salvage yard is a good place to get a steel rim for this use. Then clean the rim very well and give it a coat of Rust oleum paint, then have the tire mounted and balanced on it. When you put the spare in place be sure to hang it so the valve stem is down so you can check its pressure. I see a lot of flat spare tires and that makes them pretty useless.

Thanks Steve, I am going to my Dealership tomorrow and will ask them if my 275 tires will fit on to my 245 spare rim.

They will fit but not correctly because the spare rims are normally only 6-7" wide while the normal rims are 8-8.5" wide for those 275 tires. The bigger issue with the spare is that it needs to match the diameter of the other tires. That is why they use the narrower rim, it bulges the tire out, so it matches closer.

Thanks Steve. So, I was thinking about getting a 245/70/R18 Michelin tire for this spare rim. The temporary spare is a Goodyear 245/70/R18. I have lost faith in the factory tires on my new 2021 truck, thus I am replacing them with Michelin's. Another concern about the spare tire is will the 245/70/R18 the only size that will fit under the truck properly. That is, the ground clearance and the wheel diameter is proper for that size tire? These concerns lead me to believe a 275/65 R18 it's proper rim would not fit properly to factory spec's for the spare tire? If I can put a Michelin tire on as the spare, it would be a much better tire, and I feel I would not have the limitations that a temporary spare has on it. That is it should only be driven 50 miles at lower than normal speeds to a shop where I can get the blown-out tire repaired to replace the temporary spare. Thanks, Jim Felland

Thanks Steve, So I was thinking about getting a 245/70/R18 Michelin tire for this spare rim. The temporary spare is a Goodyear 245/70/R18. I have lost faith in the factory tires on my new 2021 truck, thus I am replacing them with Michelin's. Another concern about the spare tire is will the 245/70/R18 the only size that will fit under the truck properly. That is, the ground clearance and the wheel diameter is proper for that size tire? These concerns lead me to believe a 275/65 R18 it's proper rim would not fit properly to factory spec's for the spare tire? If I can put a Michelin tire on as the spare, it would be a much better tire, and I feel I would not have the limitations that a temporary spare has on it. That is it should only be driven 50 miles at lower than normal speeds to a shop where I can get the blown out tire repaired to replace the temporary spare. Thanks, Jim Felland

They built that same design with a full-sized spare as an option with most trim levels so that shouldn't be an issue. I would just hunt up a steel rim and put a tire on it. Or look at the rim you have and see if it is an 8J rim. That will be stamped on the rim. It means it's an 8" rim and should handle the new tire just fine. The normal temporary spare on those is a narrow tire. Which trim package is your truck?
If you wish you can visit https://www.cars.com/research/ram-1500-2021/trims/ and look at the options, they had on those trucks. It will show you the original rim sizes and what spares could be had but if yours has 8J the new tires would fit it. I do suggest a steel rim for the spare, because of the rarity of actually using the spare tire these days a steel rim tends to stay inflated better than the aluminum rims.

Thanks Steve W.,
My trim line is a Big Horn, 4WD, Crew Cab, 5.7L, with four 275/65/SR18 116T tires. The web site you sent me to has listed the spare tire as "full size" (not matched full size),but is actually 245/70/R18 Temporary Spare Goodyear.
The website says the four tires are 18-8 but no listing for the spare other than steel material.
I looked at the spare rim that is on the truck for a stamp of "8J" but did not see it on the side facing out. I did see stamps saying A ...14 21, ET19.05, MAXION 26, 5YD43TRMAD, 04755196D.
The HUB this Rim slides over has numbers T38, MW.
The center metal core of this hub has numbers 113A, 810?25A.
Hope this helps.
Jim Felland

Yep, that is the full sized spare they used. It is a 7.5" width rim. For the tires you want you want an 8" wide rim. Still will fit in place of the one you have though.

Thanks Steve,
When getting the new tires put on, I had them try to put one in the place where the spare goes under the bed. It fits properly and safely! So, I ordered the fifth tire, a steel 8" rim and a tire pressure valve stem sensor for the spare I want. Yeh!

Sounds like a plan. Thanks for the update.