How To: Choose an E30 differential, tools and guide

I recently bought a 6-speed transmission out of an E60 530i to put into my M42 E30 with a 4.1 diff. Knowing that the gearing is higher with the new transmission I decided I needed a new diff. There's a lot of info online about diffs, transmissions, and wheel sizes for E30s but it all has to be pieced together. Here's what I did and may be helpful;

---

1. I created a spreadsheet with all of the common E30 transmissions and swaps, that gives you the gear ratios for each gear. To use the spreadsheet, just go to this link, and select your transmission from the dropdown.

---

2. Now that you have the gearing for your transmission, go to this site and plug in those numbers.

---

3. Set your units to MPH

4. Use the calculator at the bottom to get your tire diameter

5. At this point you can enter in your current and new diff ratios, then compare old vs new. Based on this, I think I'll need a 2.93 diff, or maybe a 3.23