If you have the original rear end it is more than likely a 3.90.
And if you are using a non overdrive automatic (like a 350/400)you can't expect to get much mileage out of it anyway. You can do some good by tweaking the carb & timing but it's still geared low. Put the trans in neutral, jack up one wheel, mark a location on the tire and the driveshaft, turn the wheel two complete revolutions and count the number of turns on the drive shaft. Three & 1/2 turns = about 3.55, almost 4 = 390, a little over 4 = 4.10 etc.
Fred
