5.13 is still small for a D30, to me. From what I have seen there is barely over 1 gear tooth in contact between the pinion and the ring. Turning heavy 37's is just asking for trouble. I was looking at the 5.13 but I'm running small 35's with. 3.8l. I will probably go 4.88 as a compromise between the rpm's and R&P gear tooth size but with a 3.6 I could see the 4.56 as a viable option.