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Tactically I would consider letting the engine idle when you can and using DFCO when necessary.
Trying to balance keeping the engine at operating temperature (just barely) with idling and DFCO (which will cool it off and make it run more rich).
Engine off will just make it drop below operating temp which will bring on enrichment. In essence an additional cold start.
Minimise cabin heat as much as possible without risking your safety and some semblance of comfort. Recirculate the heat if you can without excess window fogging. Fresh air requires much more engine heat to get it hot enough to be effective in extreme cold.
You objective should be to balance DFCO and idle to create just enough heat to keep the engine warm enough to operate without enrichment.
A scangauge will give you the fuel consumption data to verify your fine tuning your tactic. It may even be better to use lower gears for shorter times, versus higher gears for extended periods of time, since the longer periods of no fuel consumption could get your engine temperatures to low and increase the mixture.
It is a fascinating scenario, where the eventual tactic may be somewhat unusual. Your idle fuel consumption is probably in the range of .16 to .22 GPH. Make sure the idle speed is as low as tolerable. If you can average 50 MPH with 1/7 gal per hour consumption, you are getting 350 MPG. While not as good as infinite, you will do better when the grade levels out and you have a warm engine. It's a balancing act where you burn the least fuel to try to keep the engine warm.
How long is the downhill ride as far as time?
Keep us posted as to your progress.
regards
Mech
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