340 Gets Terrible Gas Mileage

If you can't figure the rear gear any other way, get a GPS and take accurate speedo and tach readings. There are online calculators. Another way is jack up one rear tire with front safely blocked, put trans in N. See if you can turn wheel, and make certain you have the parking brake "off.

If you can turn the wheel you do NOT have limited slip, and do this: Turn the wheel exactly 2 turns while counting driveshaft turns. About 3 1/4 turns is 3.23, etc. If you can NOT turn the tire, jack up BOTH rear tires

Now, turn one tire only 1 turn exactly and again, count driveshaft turns. Same deal as before