Transmission replaced, mpg way down

The transmission was shifting strangely and
skipping gears at 89,000 miles. I was told I needed
the transmission replaced. I opted for a used
transmission from a reputable auto parts place
(LKQ in Wisconsin) rather than a remanufactured
unit since I had planned on selling the car this
spring. The new to me transmission has 63,000
miles on it and a brand new torque converter was
put in, both by the Ford dealer. The new
transmission shifts like the old did a few months
ago when I suspected something was amiss. The
dealer test drove it and said it is normal. I got
24mpg highway prior to the replacement. Driving
the same highway route immediately after
replacement 4 weeks ago and to this day, I am
lucky to get 18mpg highway. The dealer has no
answer for that. How can I lose 6+mpg from a
transmission change?