The sock contains about 100g of silica gel. I keep it on the dash for a couple of days and then on a radiator at home over night, this is sufficient to keep ice from forming on the inside of my windshield, saving me from having to idle the car before driving off.
On really cold days i think i'm saving about 5 minutes of idling twice a day. @6 days a week that's and hour, which translates to over a gallon per month. (maybe 1.5)
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2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)