As it stands, diamond is actually a downgrade from gold. Gold is a 1:1.5 ratio of fuel to speed while Diamond is 1:1.166̅ then obsidian is 1:2. I think the intended stat for diamond was 2x fuel cost instead of 3x which would have made it 1:1.75 and right in between the two. This might not be accurate to the in-game speed but the tooltip says 3x.