using a 240 volt 80 amp tesla wall unit. My old modelS with dual chargers would charge using 40 amps on each charger at 66 to 68 miles an hr. My model X with 72 amp charger puts on 38 miles an hour. I do not understand why the rate is not higher and certainly glad I got the 72 amp charger

Make sure the car is set to allow 72 amps. Make sure the car is able to set to 72 amps. If car tops out at 48 amps you may need to talk to a SvC to update you for 72 amps. There are a few threads with "lots" of people that paid for 72 amps but got 48 amps.

At 72A we get 46mph charge rate. We get 60mph at 80A in the S. 72 is 90% of 80, so shouldn't we get like 54mph in the X?

Also note that the charge rates in MPH are different for the Model S and X. (34 vs.30 at 48A and 52 vs.46 at 72A)

"Slower" charge on the X because the X uses more KWh per mile. Same KW going into the car is fewer MPH going into X.