Keith_McClary wrote:... assuming my time is worth $X/hr and gas is $Y/gal, I can calculate the optimum speed.
Maybe tomorrow I will do the math.
OK, I did the math.
Suppose you are paying someone $R/hour to drive and you are also paying for the fuel (assume no other expenses). You don't care how long the trip takes.
If fuel is expensive you would want to stay at the Sweet Spot speed. If it's very cheap you want to go faster to minimize the payroll. For intermediate fuel costs there is an optimum speed that minimizes cost of the trip. I get the approximate formula (good near the Sweet Spot):
E = 0.63 R/C
where:
E = optimum increase of speed (above Sweet Spot speed).
C = fuel cost per unit distance at the Sweet Spot.
The (dimensionless) number 0.63 is essentially the curvature of the graph at the Sweet Spot.
For example, if C = ($25/gal)/(25 mpg) = $1/mile and R = $10/hr then
E = 0.63($10/hr)/($1/mile) = 6.3 mph.
If fuel is "only" $4/gal then E is about 40 mph (the formula is inaccurate at these values).
My conclusion is that fuel costs would have to be several times higher before the cost of driving faster exceeds the the value of the drivers time. So that's another reason they speed.