School me on torque/stall converters

in my 66 i have an 87 gran fury tf904 with lockup and an aftermarket converter thats approx 1800 stall, 3.23 gears, zero decked 318 with 360 top end. On a 500 mile roadtrip to spokane washington from missoula montana i averaged 18.4 mpg doing 65-70. granted i have lockup, the stall is nice to weave and bob like a jackass through traffic and is peppy enough to have a little tire barking fun here and there.