A midsize gasoline sedan that gets 45 miles per gallon at a steady 55 mph falls to 38 mpg at 65 mph and to 32 mpg at 75 mph, according to Oak Ridge National Laboratory data. Between 55 and 75 mph, that is a loss of 13 mpg, or roughly 29 percent of the car’s highway efficiency, for a trip that saves a driver about a quarter of the travel time per mile.
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The numbers come from a federal data set rather than from one test car, and they describe midsize sedans as a class. Diesel sedans lose even more between the same two speeds, 18 mpg against 13, and hybrids lose 13 mpg like the gasoline car, and trucks and SUVs with larger frontal areas are likely to pay a bigger penalty.
The Oak Ridge table for gasoline, diesel and hybrid sedans
The figures sit in the Department of Energy’s Alternative Fuels Data Center, which publishes fuel economy at various driving speeds and credits the Oak Ridge National Laboratory’s Transportation Energy Data Book, edition 39, Table 4.33. For midsize cars, conventional gasoline reads 43 mpg at 45 mph, 45 at 55, 38 at 65 and 32 at 75. Conventional diesel reads 57, 55, 45 and 37. Hybrid electric reads 55, 46, 38 and 33. The data book is produced by Oak Ridge for the Department of Energy’s Transportation Technologies Office, and its current edition is edition 41.
Carscoops, which walked through the same table, adds one more detail: a hybrid’s best figure comes near 45 mph, at up to 55 mpg, and falls to 33 mpg at 75 mph. The page does not state units on its columns, so the speeds in miles per hour and the economy in miles per gallon are read from the table’s labeling and the Carscoops description.
Aerodynamic drag grows with the square of speed
Carscoops attributes most of the loss to aerodynamic drag, which rises with the square of speed. A simple calculation shows the scale: 75 divided by 55 is about 1.36, and squaring it gives about 1.86, so the air pushing back on a car at 75 mph is roughly 86 percent greater than at 55 mph. The engine has to supply the extra work, which is why the mileage curve bends downward so quickly past 55 mph.
The federal consumer site fueleconomy.gov says gas mileage usually decreases rapidly at speeds above 50 mph, and treats each 5 mph driven over 50 as like paying an extra $0.21 per gallon of gasoline. The Department of Energy’s Energy Saver guide uses a higher $0.27 per gallon for the same 5 mph, tied to its own gas price of $3.83, and puts the cost of aggressive driving, including speeding, at about 15 to 30 percent of mileage at highway speeds.
What 20 mph is worth over a year of driving
Carscoops converts the table into dollars: a driver covering 15,000 highway miles a year at $4.25 per gallon for regular gasoline would save as much as $576 by driving at 55 instead of 75 mph. The arithmetic checks out. Fifteen thousand miles at 45 mpg takes about 333 gallons, and at 32 mpg it takes about 469, a difference of roughly 135 gallons. For diesel at $6.35 per gallon, the same comparison, 55 mpg against 37, saves more than $800.
The diesel column shows the steepest slide in the table, from 55 mpg at 55 mph to 37 mpg at 75 mph, a drop of about a third. The hybrid column is the most unusual, because hybrids peak at 55 mpg near 45 mph, and then converge with the gasoline car: at 65 mph both read 38 mpg, and at 75 mph the hybrid leads by a single mile per gallon, 33 against 32. Carscoops notes that no electric-vehicle data accompanied the table, though range also drops at higher speeds. For anyone comparing powertrains, the table’s message is that every type tested loses efficiency between 55 and 75 mph, and that the gasoline sedan’s 13 mpg slide matches the hybrid’s and is smaller than the diesel’s 18 mpg.
Those are best-case savings, and they assume every mile is driven at a steady highway speed. The Energy Saver guide estimates that observing the speed limit improves average fuel economy by 7 to 14 percent when drivers slow by 5 to 10 mph. The same guide lists other drag and weight penalties: a large, blunt roof cargo box can cut fuel economy by 10 to 25 percent at Interstate speeds of 65 to 75 mph, and extra weight costs about 1 percent of mileage per 100 pounds.
The open question is how closely a particular car follows the table. The Oak Ridge data describe average midsize sedans, and the page behind them gives no publication date for the dataset.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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