Pick your cover by overall tire diameter for the right fit. Choose white if you prefer a light color. If your tire sits at the low end of a size range, expect a looser fit; choosing the tighter range can snug things up. In windy areas, look for options with grommets or a draw cord, while snap-on ring styles slip on and off fast.
They can, when sized and secured well. A snug fit with elastic around the edges helps reduce movement. Use rear grommets, a draw cord, or a simple bungee to add security in high winds. Note that some draw cords require reaching under the trailer to fasten, so choose a setup you're comfortable installing.
Match the cover to your overall tire diameter. For reference, 205/75R15 is right at 27 inches, 215/85-16 is 30.4, 215/75-17.5 is 30.2, and 305/75R22.5 is 39.5. If you're at the low end of a size range the fit can be loose; a next-smaller range can snug things up if it still clears the tire. If the cover puddles on the ground, consider a shorter option or adjust for some clearance.
Installation is generally easy. Slip the cover over the top and pull it down; snap-ring and elastic-edge styles go on and off quickly. If instructions seem clumsy, follow the pictures and take your time. Aim for a fit that's not dragging on the ground to let water drain and air flow, and add a rear bungee if you want extra hold.
White can be better with the heat versus dark gray. The lighter color may reduce heat buildup on the tires in sunny conditions. If heat is a concern where you park, leaning toward White is a simple way to help. Still check fit and securement so wind doesn't compromise coverage.