If I were looking at diesel I'd be looking at the whole package, not just fuel consumption. Notwithstanding the post above, one thing that doesn't get highlighted that much is the propeller. Diesels vibrate and otherwise durable aluminum propellers apparently can't take it. Diesels also have a narrow power band so you need variable pitch. The end result is a relatively fragile wood prop (MT on the Thielert) or a carbon fiber prop (apparently a Hartzell on the 182) that's better than wood, but still not as resistant to hangar rash as a fixed pitch slab of aluminum. As with everything diesel related, the 'solution' is fussiness, much increased complexity, and increased maintenance.