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1999 BMW 323/Subie Impreza Questions

Dear CarTalk,

I have two cars and two questions. My first car is a 1999 BMW 323 with a manual transmission. My question is: if gas consumption is based on RPMs (as you stated in a show many moons ago), when I downshift and the RPMs go up, but my foot is off the gas, is it still using more fuel?
My second car is a 2019 Subaru Impreza. This car has all the nanny safety features, including adaptive cruise control which I love. When I get within the safety range of the car ahead it will slow down to match its speed. When it does this it feels like its dragging a drogue. My question is: Is it riding the brakes to create this effect and is it hurting my gas mileage?

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