SPEED is a relevant thing but if your going to slow your actually an impediment to traffic and a hazard, if you go to fast your risk tickets and buring more fuel. In my truck I can run 68 all day long and get over 10MPG with a standard load of parts that weighs under 4000 lbs...now start adding weight, WIND, stop and go city rush hour and all this changes.
I think the best speed is the safest speed you can effectively maintain for the longest average time...changing speed, mashing the gas for wind, or hills or passing all the time, gonna use fuel...
I always try to fit "INTO" traffic, go with the flow, use a TT for a wind break...whatever will give me the edge and save fuel. But some days you must use the HYPERSPACE FREIGHT button and go 73...