What is the OEM rear end gear ratio on a 1979 ford F-150 400 9-inch end?

There's no single answer to the OEM rear end gear ratio for a 1979 Ford F-150 400 with a 9-inch rear end. Ford offered a variety of gear ratios depending on the truck's options (like transmission type, intended use, etc.). To find the ratio for *your* specific truck, you need to look at one of the following:

* The vehicle's data plate: This is usually located on the driver's side doorjamb. It may list the axle ratio.

* The differential tag: This tag is usually located on the differential housing itself. It will clearly state the gear ratio (e.g., 3.50, 4.11).

* The owner's manual (if you have it): This might list the specifications for your truck's axle.

Without one of these sources, it's impossible to determine the exact gear ratio. Common ratios for that era and engine might include 3.50, 3.73, or 4.11, but this is just speculation. You need to check your truck directly.