That'* what my "Jimmy" manual says, the GMC Jimmy, that is!!! :P
It'* suggested to use D on hills or when towing. I don't tow.
Gas seems to last longer around the Seven Hills of Seattle when using D.
On our trip up to Whistler last year, I used OD and/or Cruise.

I got an answer to the question this last weekend.

I took "Martha" (...I named the SSE in honor of my MOM! ...) up to
97BonnSSE'* place.
66 miles each way.
Up in D.
Back in OD.

Results:
Up = 15.9 mpg. ~2500-3000rpm/65-70mph
Back = 24.7 mpg. ~1700-2000rpm/70-75mph

BIG difference.

Next time, I'll try both directions in OD; one with cruise, one without.
I love this car.

97bonnSSE lives WHERE? Another Northwester around? Have him post in your gatherings post.

Did you try in reverse?

97bonnSSE lives in Burlington.
I'll send him the link to the Gatherings post.
Maybe Turtle and I could meet at 97bonnSSE '* place.

Reverse?
Now THERE'* an idea!

I had a couple of miles to go to get to the freeway, but, it was mostly freeway driving.
There was a LOT of rain that day and some traffic was slow, both directions.
I don't know when this car had a tune-up last, and maybe the 'Firehawks' aren't conducive to good mileage.
I got on it a couple of times, and didn't use cruise control.

I'd like to see better #'*. What might they be under ideal conditions?

Even in my modded SSEi, I'm doing better than your freeway mileage. How old is your O2 sensor? And have you checked your plugs lately? Might be time to do some poking around in there.

I find my gas mileage is close with od getting a little better, the biggest differences I find are with acceleration and decelleration. With OD it hardly accelerates and when i let off the pedal my SSEi coasts forever. In D it immediatly responds when i stomp on it but when i let off it feels like i am applying the brakes and when i accelerate again it jumps a bit.

I've only had this vehicle a couple/three weeks, and, it'* been raining most of that.
I don't have an empty garage to poke around in, either.

Don't know about the O2 sensor or plugs yet. The low mileage on the car makes me think I should replace both items as a matter of course. It might have sat a LOT or for extended periods. Maybe that won't hurt either of those parts, but...

Isn't this a Series I engine in a '92?

You are Series 1. It ran from 92-94 for the SSE and SE.
Series 1 for the SSEi or SC SSE or SE went from 92-95.

My car sat in storage for almost 5 years. During that time, the O2 went out and the plug gaps eroded to .090". Spec on the gap is .060". I'd say you're due, but do the wires, too.

Ah, so!!
(...for the younger guys, that'* "Ahhh...., so....!", not, "...***---hole..." ! ) :P

Just waiting for the bundle of ca$hflow.
I expect to be amazed.