1982 W123 300TD. Solid maintenance. Regapped valves 6,000 miles ago, when I also had all filters changed.

Change oil every 3,000 miles.

I was getting 30-ish mpg on the highway, 26-ish in the city for a 28 overall mix since I bought the car.

Then about a month ago, the mileage between fillups started dropping like a rock. 24. Then 25. I just went to Salt Lake and back, and got 24, 22 and 23 on my tanks.

What gives?

Some variables I've considered...

Salt Lake is at high elevation. It's started to get kinda cold (car still starts fine, though).

One thing I did notice on my last tank fillup was some leakage out of the top of the tank cap. The cap is locked solidly, but it appears that while I'm driving for the first 90 miles or so on a fresh tank some diesel is leaking out of the cap, somehow, and down into the drain hose. There is also some splattered back on the right rear fender. I hadn't checked that when the mileage started dropping, so it might have something to do with it. But how is diesel leaking out and up through the gas cap when it is locked?

Any help is appreciated.