At which outside temp should I decide to drive my other car?

I ask becAuse while I have a block heater, I can't plug it in at work.

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1982 240D 4-speed
2013 Jeep Wrangler Sport - 4.10 e-locker

That depends on the starting performance you get without using the block heater. This in turn depends on a number of factors already mentioned in this thread. If you're getting into temps where starting becomes iffy, then it's time to take the other car, assuming it has better cold start performance.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.