CREATE TABLE table_6637 ("Series Ep." TEXT, "Episode" FLOAT, "Segment A" TEXT, "Segment B" TEXT, "Segment C" TEXT, "Segment D" TEXT)

What episode has a manual motorcycle transmissions section D?
SELECT SUM("Episode") FROM table_6637 WHERE "Segment D" = 'manual motorcycle transmissions'
SELECT SUM("Episode") FROM "table_6637" WHERE "Segment D" = 'manual motorcycle transmissions'