Not to be antagonistic but what gasoline engines which are not a hybrid get (US or Imperial) 50 to 60mpg? I could see diesel engine vehicle average mileage for small manual transmission getting high Imperial 60mpg gas maybe high 40s mpg.
Deep water and gasoline engine don't tend to work well, and in my region snow gets under the hood then melts (can't get it all out) which causes problems.
Compared to a gas engine a diesel engine will run underwater if it has air you see 4x4 trucks with snorkels on the A-pillar (and some other sealing) in Africa countries, Australia and military use too.
* I get about 1.25x the mpg in the diesel to her in the petrol. 1.5x compared to myself in the petrol (no mpg quotes because it depends on the type of driving)
* The torque is useful. I drive a 30 minute commute, mainly motorway. If I drive hers it is a noticably worse drive.
* For in town driving hers feels better every time. Easier to handle with no load (hauling something? Use the diesel), the diesel tricks me into speeding.
* Wet weather? Never had weather problems in either car.
So whilst I am definitely a less efficient driver than her I save a significant amount on fuel by having a diesel, and my calculation shows its several hundred a year better than if I had bought the petrol when I had the chance - more than covering the increased cost of the initial purchase.
Diesels better in wet weather, really? I've never owned one, but neither have I ever had to leave my petrol car at home because it was raining, and I've owned some fairly old cars :)