The Department of Energy has published essentially the same chart for thirty years: a typical car's fuel economy peaks somewhere between 40 and 55 mph, then falls off a cliff. At 75 mph, most cars burn 15 to 20% more fuel than they do at 60.
Why speed matters more than everything else on the highway
Aerodynamic drag rises with the square of speed. Double your speed and drag quadruples. At 30 mph, air resistance is almost irrelevant — your engine is mostly fighting rolling resistance and internal friction. At 75 mph, drag accounts for over 60% of the fuel your engine is burning.
This is why highway MPG numbers collapse above 65 mph. You're not going 15% farther in the same time — you're shoving 30% more air out of the way per second.
The $0.25-per-gallon rule
The DOE's oft-cited rule of thumb: every 5 mph you drive over about 50 mph adds the equivalent of roughly $0.25 per gallon to your fuel cost at 2026 prices. Driving at 75 instead of 60 is like driving a car that gets the same mileage — but paying $0.75/gallon more every fill-up.
For a long highway commute that's significant money. A driver doing 15,000 highway miles a year at 28 MPG pays about $2,000 in fuel. Dropping from 75 to 65 on the same route saves roughly $300.
The time-value trade-off
Slower speeds save fuel, but they cost time. On a 30-mile commute, dropping from 75 to 65 adds about four minutes each way. For most drivers, that's not a trade worth making daily.
The calculation flips on long trips. A 600-mile road trip at 75 takes 8.0 hours and burns about 21 gallons. At 65 it takes 9.2 hours and burns 17 gallons. You're paying $16 in extra fuel to save 1.2 hours — roughly $13/hour for the time, which is worse than minimum wage in most states.
Where cruise control fits
On flat highways, cruise control saves fuel because humans unconsciously vary their speed by 2 to 5 mph. That variance is tiny fuel losses compounded every minute. Setting the cruise at a lower speed captures the benefit of "slower" without the constant vigilance.
On rolling hills, cruise control can hurt because older systems downshift to maintain speed uphill. Modern adaptive cruise is smarter. If your car is a 2015 or newer, trust the cruise. If it's older, manual throttle on climbs is usually better.
Diesels and EVs have different curves
Diesel engines keep their efficiency better at high speeds because they run leaner. A VW TDI or a Chevy Duramax can see highway economy drop only 5 to 8% from 60 to 75 mph — much less than a gasoline car.
EVs, on the other hand, have some of the steepest speed penalties on the road. Because there's no transmission losses to hide behind, drag is the dominant energy cost. A Tesla Model 3 can lose 25% of its range going from 60 to 75 mph. EV road-trippers who slow down ten mph routinely gain a full extra charging stop of range.
The takeaway
Highway speed is the single most direct lever you have over your fuel bill on long drives. You don't need to crawl. You just need to know that every 5 mph is costing you roughly a quarter per gallon, and decide whether the time saved is worth it for the trip you're actually taking.
