Camryglide - '20 Toyota Camry hybrid LE 90 day: 65.83 mpg (US)
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I mounted my antenna on the inside of my windshield so it contributes zero aero drag. The relocation produced only a slight drop in reception sensitivity.
De Yaris - '04 toyota yaris T2 90 day: 69.55 mpg (US)
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I'm thinking that if you have a standard car then it'll make no measurable difference. But if you have a highly modified mpg machine and are already getting galactic mileage, then it may make a few mpg appear.