If it has stock-type tires, a 3.08 should work good for economy, and a 3.50 for power.
1.5:1
You can refer to your user manual to ID the rear end gear ration in 80 Chevy pickup.
Supply the vehicle identification number to the nearest dealer and he/she can tell you what ratio you have.
What diameter tires are you running? Do you want... Highway Driving, Good Fuel Economy? Daily Driver, Best Overall Performance? Most Power and Towing, Reduced Fuel Economy? All these questions need answered first, then you can make a good choice.
Need to know what size engine it has to answer correctly.
It depends on the gear ratio of the rear ends. You can find this out in the glove compartment of your truck. It should tell you there. Hope this helps you out.
I would be suprised if you have adjustable valves.
Depending on the differential gear ratio, anywhere from 17 to 19 mpg.
largely depends on axle ratio
9.4:1
What is the gear ratio for a 1985 CHEVROLET K10 pickup
3.08 makes a good ratio for economy. 3.55 for more power when towing.