That is fine when needs must. I did the same when my exhaust broke off and was dangling below the car, but to save a couple of miles per gallon I would just be selfish. I do drive at these sort of speeds, if there is no traffic behind me, but as soon as traffic comes up behind I speed up to what would be considered acceptable for the road.
We don't have a shoulder on our roads like you do in the States, except on Motorways, where you can only pull over and stop in an emergency. You cannot drive on them.
This is typical of the roads around here.
