A system for controlling a vehicle drivetrain in a fuel-efficient manner includes a control computer operable to define a contour from substantially zero engine load to substantially full engine load, wherein the contour may correspond to a fuel-efficient path from no-load to full-load engine operating conditions. With change gear transmissions, the control computer is operable to control transmission shift points about the contour. With continuous variable transmissions the control computer is operable to modify the effective gear ratio thereof to maintain engine operation on or about the contour. In either case, fuel efficient operation may be optimized.