That depends on the speed you travel at
Because the land rover is heavy the wind resistance does not affect it as much as say a luton transit.
Rolling resistance plays a bigger part.
Dont know the figures exactly but the wind resistance begins to affect a luton transit above about 25 mph where as a fully loaded artic aint affected until about 50 mph.
you work it out
suffice to say that if you are trundling about local then the roofrack will make no difference.
I did a trip to Glasgow with car trailer to pick up a vehicle and bring it back. I removed the roofrack but it made no difference as I travelled between 45 and 55. Still only did 9mpg round trip.