I've been gathering some quotes for converting my car to duel fuel and there's 2 options;

A donut tank or a cylinder.

why does the donut tank cost so much more? what are the benefits?

a doughnut should be cheaper to intal but cost more to buy

Is the design just harder to make? I gathered a donut tank would be a customized tank and therefor more expensive than a standard cylinder. But are there things like capacity, clearance etc that make them better?

in europe they use a lot of doughnut tanks. I just think there are not many aussie manufacturers. Mine has a cut down petrol tank and a 90L gas tank in a custom tray. This used to be the common system years ago. I think they are so busy now with the rebates they want to keep the installations simple so prefer the doughnuts or multiple tank set ups. I am kicking myself for not picking up a set from the wreckers a couple of months ago. I saw both a so called "scuba" set and a cut down petrol with a tray that I could have gotten for my project fairmont wagon.