It's literally a load of bullshit. I don't really get why diesels are more to insure than petrols and I don't really get the a higher annual milage would decrease premiums (I'm not saying they do or they don't, I just don't get why)
Diesels tend to be bought by people who do enough miles to warrant them and are sensible enough to buy them not boy racers who want to rag them cos they are well fastz. With milage, surely doing less miles means you're not technically on the road as much so you are less of a risk? I do see where the higher milage and experience comes from though.
It's annoying because I want to buy a diesel and do JUST enough miles to warrant the cost of higher fuel but not really the higher cost of the car and insurance.