Would they not have to keep switching routes as a precaution? If they took the same route each time, it would make it easier for anyone looking to attack it to pick out a suitable ambush spot?
i doubt a few extra miles of fuel costs matters that much in the overall scheme of things.
geography dictates that there are only a couple of realistic routes though, so they'll be not much variance
far better to just make sure that your convoy is suitably protected - cos if some nutters are crazy enough or well organised enough to want to attack something like this they can surely rustle up enough men to be able to attack on both routes