Just tossed some gear ratios into an online calculator. My 40mph 4th gear crawl up the mountain w/ 31" tires and 4.55 gears was turning that poor engine at ~2000rpms. Poor little diesel, no wonder she was struggling.

If that was only 2k rpms and she's SUPPOSED to spin faster than that normally, this thing is going to need a muffler. The noise gets pretty ridiculous.