I generally use the rule 6",8",12" and am really carefull with the small truck and 12" of ice. Would prefer about 14" or more. Its not worth your life for a few fish. Also, your truck will make a ice wave infront of it and if you go too fast and pass the wave it is a good chance the ice will break.
Thats why ice roads up north have about 30 mph speed limits, and large trucks stay about 1/2 mile apart or more. And were talking 4 to 6 FEET of ice or more.