Let me get you some pics! Keep in mind in these pictures I'm running the same 17x8 american eagle alloy wheels...So they are pretty narrow. Also I'm running 4.56 gears
Here is my current setup with 37x13.50x17 Toyo M/T DC lift cranked about 1.5" my fenders are @ 47" up front. Angles are great, ride is perfection. Keep in mind these Toyo's measure out to be a TRUE 37". I like 4.56's with these 37's better. I lost a little take off speed but gained top end speed and lower RPMs on the highway. Before this on 35's above 60 mph it sort of ran out of gear and would only top out at 80 or 90 @ a higher RPM...
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Here I am back on my BFG 35x12.50x17's...Same lift specs DC + 1.5" crank TOWERING these BFG's...Kind of miss the 35's less weight, quicker truck 0-45, more mpg's. Keepin mind these BFG 35's measure actually a bit small AND less narrow than 35x12.50. 4.56 gears was faster than stock up to 50 mph but like I said lost top end speed above 60 but very quick up to 60 mph.
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If you have any questions, lemme know!! Based off my pics the biggest you could go is probably a true 38" tire, but that will probably depend if you have a narrow wheel like mine or a wide wheel with more offset you might not be able to.
Here's a quick gearing chart that I've always seen
Now this chart is considering you're running tires that truely measure 33 35 37 and or 38". Like my bfg 35's actually measure 34.4 or something and weren't actually 12.5" wide at all.
33's = 4.10
35's = 4.56
37's = 4.88 (although I love my 37 and 4.56 combo)
38's = 4.88
40's + = 5.13 and up