Switching from a manual stick shift to automatic transmission.

[no_hypocrisy]

Will I lose mileage (mpg) with an automatic?

Both Honda Accord sedans.

[hlthe2b]

1. Some, but probably not much and you can limit it with a more conservative driving style...

[crud]

2. a lot of automatics have a manual mode

You can click through the gears

[multigraincracker]

3. Modern 6 and 7 speed autos

can do better than manuals now.
I just can’t stand them.

[House of Roberts]

4. Here are stats for all years in one article.

https://www.vehiclehistory.com/articles/honda-accord-gas-mileage

In later years the difference seems to favor the automatics, with the CVT models even better.

[paleotn]

5. I did when I switched to an automatic Subaru.

Roughly 4 to 5 mpg, but that's with AWD. Type of automatic and driving habits play a part, so your mileage may vary.

Had to since manual Subaru's are becoming scarce. A year in and I'm still occasionally feeling for the clutch. My left foot has such a boring life now. Until now, it's known nothing but manuals since I learned to drive in the mid 70's.

[WmChris]

6. Maybe not

2020 Honda Accord 40 might with cut transmission.

[Jirel]

7. New car? Usually not.

Most new cars are made to be as efficient or better on mileage with automatics versus our best efforts with manual.

[no_hypocrisy]

8. Moving from a 1999 to a 2010

[Stuart G]

9. K & R