A number of the tire stores and sites have various wheel size and tire size options for your specific application.

I do a lot of research on TireRack.com . They have numerous reviews and track tests. I assumed it was an Explorer Sport 4x4 with a 20" OEM wheel, but you didn't specify. Let's see if this link works:

https://www.tirerack.com/wheels/resu...oModClar=Sport

Just a quick look indicates that there are a bunch of options in an 18.

I've also used DiscountTire.com to find info and I've often bought through them. Discount did have the incorrect OEM sizing for my 2019 Ram, so make sure you double-check the info to make sure.