As with many OE parts, stock transmission pans and differential covers have a basic design based on function, ease of manufacture, and cost. But aftermarket trans pans and diff covers are designed to increase transmission and differential gear performance under tough conditions, with cost and ease of manufacture of secondary concern. Basically, aftermarket transmission pans and differential covers improve performance in three ways: through increased lubricant capacity, by improved cooling of the lubricant, and with stronger, smash-resistant construction. Not mentioned, but worth noting, is the appearance of cast-aluminum trans pans and diff covers - they simply look way better than stamped steel OE pans and covers. Here's a sampling of aftermarket differential covers and transmission pans.