That torque sounds great. I bet you could enjoy a singificantly taller gear. If the tires are the same outside diameter as stock, you're looking at 2750RPM at 70mph...that's above your peak torque and that diesel should probably be running far lower RPM.
good thing I drive 60-65 everywhere I could swap the guts from a 280zx 5-speed they have a .73 5th. That would give a nice 2200 RPM 60 mph. The 3.36 gears are the tallest Nissan offered for this housing. Economically that's as good as I can get. I could swap the H190 out of the truck the SD22 comes in for a 3.16:1 ratio but that involves brackets and cutting tubes and all that fun. I am by nature a bit lazy and I think the 3.36 will do. I should also change up to the 205/50/15 tires to have the correct diameter that would bump my cruise at 2200rpm to 62 mph
you know more than me but what I've seen these Nissan diesels like to cruise at peak torque. interesting side note is if the engine could pull to the governed 4400 rpm it would go 121 mph... Thats better than the stock engine could ever do.
Remember that peak torque RPM is only producing peak torque with the go-pedal on the floor, so we're talking RPM but not power production. If you're producing 193 lb-ft while cruising, you're probably cruising at 160mph.
Anyway, you may be right, diesels might be most efficient at/near the same RPM as they produce peak torque.