'85s are the best.
They do have the 2.88:1 gearing as said, better trans system (blue UFO), and sadly have previous year CA emission. Even so, I've been getting 29-31mpg often in my SD. I typically cruise around 63-67mph on the freeway (hey, I've got no one to see or go home to, so I'm never in a rush when out and about).
I had an '84 300D prior to picking up this SD and I like the SD far better than the D. More room sure (as much as a SWB can give you, but that's why I have the SDL too), but the SD looks better to me also. Better interior bits, power seats, electric vac system, etc. I don't think the SD is that much heavier than a 300D either, my SDL is around 3800lbs so the SD has to be around 3600lbs I'd guess.