http://www.streetsourcemag.com/forum/topic.aspx?topic_id=92832&Topic_title=20039;s+on+a+B%2FD+Mazda&forum_id=23&forum_title=Mazda+Trucks
sorry i dont feel like making it a link just copy and paste, but RUSS D's post should answer everyones questions, but just because someone has a nissan, yota, or any other mini for that matter tuckin 20X8.5's it doesnt mean anything, mazdas are the worst trucks to tuck big wheels on, unless you have a totally custom frame.
word of advice, 0 is the center of the wheel, anything positive after that, for example +25 means the mounting face of the wheel is closer to the outside face of the wheel, anything negative like -25 is closer to the back of the wheel
billet wheels are different and are always sold as back spacing and is different than offset.
remember 25.4 mm is equivalent to 1 inch, i hope this and russ d post help, russ has done the homework you need though, just copy and paste and check it out...