Perhaps your "run across the mountain" put the engine under sufficient load to blow excess carbon out of the engine. Or should I say it burns it out? I've been taking my 1984 300SD turbodiesel out on some longer drives, and on the way back I cross South Mountain while holding the transmission in "L" keeping right at the posted 40 mph speed. This puts my engine speed close to redline. Not a good area to speed since the speed limit drops to 30 mph thanks to a well-hidden sign near the top, and all the way down. By the top of the hill the temp climbs about five degrees, as well as leaving a nice cloud of smoke.
I've been enjoying improved driveability as well as a 1-2 mpg increase in mpg, and this is on the tanks where I crest a mountain in second at high rpm's. I know we have different engines but this was just a thought.
Another is the amount of time spent in the lower gears in city driving. If you're driving hard on highways, you'll be in top gear most of the time. My car averages about 19.7 mpg in city driving, but on the highway I get 25 or so.
[This message has been edited by Robert W. Roe (edited 09-07-2000).]