What is the gear ratio on a 1980 ford f100 4.9L?

There's no single gear ratio for a 1980 Ford F-100 with a 4.9L engine. The gear ratio in the rear differential varied depending on the options selected when the truck was built (e.g., payload capacity, intended use). Common ratios included things like 3.50, 3.73, and 4.10. To know the *exact* ratio for a specific truck, you'd need to:

* Check the vehicle's build sheet (if available): This would list the axle ratio.

* Look for a tag on the differential: The differential housing often has a tag indicating the ratio.

* Do a "jack-up and rotate" method: Jack up one rear wheel, mark the driveshaft and the tire, rotate the tire one full revolution and count the number of driveshaft revolutions. The ratio is then calculated from this. (This is more involved and requires some mechanical skill)

Without knowing the specific truck's build details, it's impossible to give a definitive answer.