2001 tundra 4.7L rear differential ring and pinion gear replacement

Whats up my fellow Tundrans... I've already got the ring and pinion gears out.
Ring is 47 tooth divided by pinion 12 tooth which gives me a gear ratio 3.91 It seems the only ones I find even close to that with that same size and ratio are specific for a V6 not a V8 is there something I'm missing? Also when it says 7.50" or 8.00" axle what exactly is it that there measuring. any info u can give me on this would be greatly appreciated. Thank you

 

2000 v8 automatic came with 3.91. Not sure exactly where on the ring gear it is measured, but seem to remember it was 8 inch.

 

Our rear diffs are 8.4” and our fronts are 7.5”
If you search here you will find all the ratios that came in our trucks