Another gearing question...I need help!

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I've searched and searched and read a lot but still can't quite find my answer...

On a full Supermaxx with 1/8 diffs, Ofna 51 spur, 40 series tires, and 4s lipo, what would be good gearing for 7xl and 9920 or 16016?

I've been trying to understand gear ratios but have only become more and more confused!!:C:

Another retailer told me I need something like a 50:1 ratio and that the 51 spur is too much and should go back to stock. A lot of things he told me seems suspect, so I am ready to start over from scratch with advice from here instead. ( I only went there cuz he had the 9920 in stock, but I wish I didn't)

I have 10, 12, 14, 15, and 16t 5mm Mod1 pinions and only a 16t 32p 5mm pinion -- what would be the best choice? I have 51t Mod1 spur and both 66 and 72 32p stock spurs. I would like to stick with the lipos if possible, and am more interested in durability than hi speed. Wheelies would be fun too. I will either use ubec or receiver pack, and can go back to standard size wheels if needed.
Thanks so much for your help!
--Jonathan