Rear Gears for gas mileage

You need to find out what gear you have in there now. Also, what's your engine combo?
This is what I know, a 1970- 340 engine 30 over, street thumper cam, j heads 2.02 intake and upgraded1.94 exhaust valves. Also has an, Air-gap dual plane intake with a 600 cfm Holley 4 barrel carb. The trans is a 727, with reverse valve body and a 2800 stall B&M torque.
How do I determine what gears are in the rear now ? I'm as dumb as a stick when it comes to this stuff.