What other costs are you thinking of, regarding "running a car longer"? I think the costs for the trip actually increase, too, as the wear on the car is greater for a shorter trip at high speed than long trip at low speed, barring exceptional cases.
The average car consumes 38 per cent more fuel at 70mph than it does over the same distance at 50mph. At 60mph it uses 34 per cent more than at 40mph. [And at 100 mph, you use 5 times as much as fuel as at 50 mph!]
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My maxima gets better mileage going 70 than it does going 55.
That's baloney. My car gets about 24 mpg at 77-79 mph and about 26 at 65-70 mph. That means that at 78 mph, I can go 240 miles on 10 gallons, while at 68, I can go 260 miles on 10 gallons. (and yes, this is actual MPG calculated over many trips by dividing miles driven by gallons used rather than relying on the inaccurate in-car computer estimate).
For that extra 20 miles, I pay in terms of time - if the total trip were 500 miles, I'd spend 6.4 hours driving at 78 mph vs. 7.35 hours at 68. I gain roughly an hour in this case, for the cost of less than a gallon of gas.
I wonder, does the government want to pay me an hourly wage for the extra time that the archaic, slower speed limit extracts from my life?