When I worked for SIA, (52 744s), they had an average burn for the fleet and at the top of each computer flight plan would be a small percentage number with a plus or minus sign that showed that particular aircraft's divergence from the average, usually shown as, for example, +1.2% or -0.56% etc. etc. and the fuel burn for the trip was adjusted accordingly.
On that fleet, (Pratt & Whitney engines), the older machines did tend to be a bit more thirsty but not always.