Diesel went over $6 a gallon in September 2026. EIA's weekly survey had the U.S. average for on-highway diesel at $6.285 on September 14, the first reading above $6 since the series began in 1994. A week later it hit $6.529, the highest weekly average in the series. That's in dollars of the day; adjusted for inflation, the July 2008 peak was higher. On September 28 it was $6.382. By then every EIA region except the Gulf Coast and the Lower Atlantic was above $6, and California averaged $8.181. A year earlier the national average was $3.754.

At $6.38 a gallon, every tenth of a mile per gallon is worth about $1,300 per 100,000 miles on a truck that gets 7 mpg, and a lot of those tenths are decided from the driver's seat.

I drive for a living, so most of this is about what a driver can change from behind the wheel: speed, idling, shifting, tires. The numbers come from federal labs and agencies (Oak Ridge, Argonne, EIA, FHWA and EPA's SmartWay program), Natural Resources Canada, the North American Council for Freight Efficiency (NACFE) and Cummins' fuel economy guide.

What speed costs you

I'll start with speed, because it moves the number more than anything else a driver decides. Air resistance grows with the square of your speed, and the power it takes to push through the air grows with the cube. For a loaded tractor-trailer at 65 mph on a level road, a National Academies review of the 21st Century Truck Partnership put aerodynamic drag at 53% of the power the engine delivers, and tire rolling resistance at 32%. That was an older high-roof sleeper, so a newer aero tractor loses less to the air, but every mph you add still makes the air's share bigger.

The rule of thumb you've probably heard is that each extra 1 mph costs about 0.1 mpg. EPA's SmartWay program calls it a general rule of thumb, and Cummins' guide applies it above 55 mph. Neither one publishes the test data behind it, but Oak Ridge National Laboratory measured something close. Between 2006 and 2008 ORNL logged six Class 8 trucks in regular fleet service, about 690,000 miles in all. On flat road, trucks grossing 70,000 to 80,000 lb got:

  • 54 to 56 mph: 7.9 mpg (only 55 miles of driving at this speed, so take it loosely)
  • 56 to 58 mph: 8.8 mpg
  • 58 to 60 mph: 8.7 mpg
  • 64 to 66 mph: 7.9 mpg
  • 70 to 72 mph: 7.0 mpg
  • 74 to 76 mph: 6.8 mpg

From 65 to 75 that is about 0.1 mpg per mph, the rule almost exactly. In fuel per mile, 75 burned about 16% more than 65 in this weight range, and about 12% more for trucks at 60,000 to 70,000 lb. You may have seen a bigger number: a 2007 American Trucking Associations report said up to 27%, citing ATA's Technology & Maintenance Council, and gave a steeper rule of thumb of 0.14 mpg per mph. ORNL's road data came out lower.

Look at the top of the list, though. The best mileage was in the high 50s, not at 55, and ORNL put the best speed for these loads at about 58 to 59 mph. The trucks were 2005 Volvos. In an earlier, rougher comparison that didn't separate loads by weight, they got better mileage at 65 than at 55, even on flat road. ORNL's "likely explanation" was that the trucks were "optimized for 65 mph highway speeds." Once you drop below the speed where your engine turns its best rpm in top gear, slowing down can stop paying.

What 55, 60, 65 and 70 mph cost at $6.38

Here's how I'd work it out for a typical truck. This is an example built from the rule above, not a measurement. Take a truck that averages 7 mpg at 65 mph, close to the 7.1 mpg the Federal Highway Administration estimates for the average U.S. combination truck in 2024 (that's a yearly average over all roads, not a 65 mph figure), and take away 0.1 mpg for every mph. Per 100,000 miles at EIA's September 28 average of $6.382:

  • 55 mph at 8.0 mpg: 12,500 gallons, about $79,800
  • 60 mph at 7.5 mpg: 13,333 gallons, about $85,100
  • 65 mph at 7.0 mpg: 14,286 gallons, about $91,200
  • 70 mph at 6.5 mpg: 15,385 gallons, about $98,200

Dropping from 70 to 65 saves about 1,100 gallons per 100,000 miles, roughly $7,000. Dropping from 70 to 60 saves about 2,050 gallons, roughly $13,100, or 13% of the fuel. That fits what Natural Resources Canada tells truck drivers: every 10 km/h (about 6 mph) above 90 km/h burns about 10% more fuel.

The 55 mph line only holds if your truck still gains below 60, and in ORNL's data on loaded 2005 trucks, it didn't. Even EPA's own SmartWay fact sheets disagree on this one. Two sheets from June 2016 put the gap between 55 and 65 mph at "up to 7 percent" and "up to 20 percent".

Slower also costs time. A thousand miles takes about 14.3 hours at 70 mph and 16.7 hours at 60, so about 2.4 hours more. If you're paid by the mile or running short on hours, that time has a price too.

Your truck will not match this list

No table can tell you what your truck gets at 62 mph. The same speed can mean 1 or 2 mpg of difference between two trucks, and between two loads in the same truck. What moves it:

  • Weight. In the Oak Ridge data at 65 mph on flat road, trucks at about 34,000 lb got 9.5 mpg and trucks at about 73,000 lb got 7.9.
  • Engine, transmission and axle ratio. Gearing sets the rpm your engine turns at a given road speed, and Cummins points out that every engine has a sweet spot where it delivers its best fuel economy.
  • The shape of the tractor and trailer. An aerodynamic tractor with a skirted van loses less to the air at every speed than an older tractor with no aero parts.
  • Wind, hills and temperature. Cummins says the season and the weather together can move fuel economy by as much as 1 to 1.5 mpg.

The only numbers I'd trust for your truck are your own. If your dash or your fleet's telematics shows trip fuel economy, run the same lane at two cruise settings and compare.

Some states set the speed for you. California caps any truck or truck tractor with three or more axles, or pulling a trailer, at 55 mph on all highways, and seven other states hold trucks below cars on at least some interstates. They're on the truck speed limits page.

Idling burns about 0.8 gallon an hour

An Argonne National Laboratory fact sheet for DOE puts a heavy truck at idle at about 0.8 gallon an hour, and estimates that a long-haul truck idles about 1,800 hours a year for rest periods, burning about 1,500 gallons. At $6.38 that's roughly $9,200 to $9,600 a year spent standing still.

The rpm makes a big difference. Cummins' fuel guide lists about 0.5 gallon an hour at 650 rpm, 1.0 at 1,000 rpm and 1.5 at 1,200 rpm, and tells drivers who have to idle for cab temperature to keep it no higher than 800 rpm. Cummins' engine manuals add a warning: long stretches at low idle can hurt engine performance, and extended idling should be done at fast idle, 1,000 rpm or more. Check what your engine's manual says. The cheaper answer is not to idle for hours at all.

The alternatives burn far less. The same Argonne fact sheet lists an auxiliary power unit at 0.2 to 0.5 gallon an hour and a diesel-fired bunk heater at 0.04 to 0.08. For heat alone, that heater burns a tenth or less of what the engine burns at idle.

Long warm-ups are mostly habit. When Oak Ridge reviewed truck and engine makers' manuals in 2016, its summary for heavy-duty trucks was that idling beyond 5 to 15 minutes is excessive. Kenworth's 2006 fuel economy paper says five minutes of warm-up is generally adequate. Natural Resources Canada says to shut down whenever you'll be parked 60 seconds or more. The exception is right after a long, hard pull: ORNL's summary says a cool-down idle is only needed after extended high-power running, and Kenworth says cool-down usually happens as you pull in to park.

Two legal points. Many states and cities limit idling. California limits diesel trucks over 10,000 lb GVWR to 5 minutes of idling at any one location (13 CCR 2485), though trucks with a certified "Clean Idle" engine label are exempt when they're more than 100 feet from homes, schools and other restricted areas. The idling laws page covers each state. Federal law also lets states allow extra weight on the Interstate for the actual weight of an APU or other idle-reduction unit, up to 550 lb (23 U.S.C. 127(a)(12)). Not every state has adopted it, and Caltrans says California has not. Where it applies, you must be able to show a written weight certificate and that the unit works.

Shifting, cruise control and hills

Progressive shifting means shifting up as soon as the truck can handle the load in the next gear, before you reach governed rpm. That's how Natural Resources Canada describes it. Cummins makes the same point from the other side: low gears are good for power and bad for economy, and more than 90% of your distance should be in top gear.

Cruise control saves fuel on a flat, dry highway. Cummins says cruise in general can make every driver better than average, though the very best drivers may still beat it. NRCan's words are that cruise "is not efficient or safe on slippery roads, steep hills or on sharp corners." Volvo says cruise control "has always been intended for use only on flat roads." On rolling hills a standard cruise keeps fueling over the crest to hold its set speed. Cummins' advice for rolling terrain is a light throttle, letting momentum carry you over short grades, and letting gravity bring you back up to cruise speed after a steep crest. Predictive cruise systems that read the road ahead from GPS maps try to do this for you. Daimler Truck says Predictive Powertrain Control on its Mercedes-Benz trucks saves "an average of up to five percent fuel" compared with trucks without it. That's the maker's figure, from European roads in 2020, and I couldn't find an independent test on U.S. highways.

Don't knock it out of gear on a downgrade. In gear, with your foot off the throttle on a grade steep enough to hold or build your speed, a modern diesel burns almost nothing: ORNL saw fuel flow drop to zero or almost zero on downslopes in its loaded trucks. In neutral the engine is idling and burning fuel, and you've lost your engine brake. The CDL manual says the braking effect of the engine is the principal way to control your speed on downgrades. Coasting downhill in neutral is also illegal in California (Vehicle Code 21710), Texas (Transportation Code 545.406), Virginia (Code 46.2-811) and other states.

Some automated transmissions are programmed to roll in neutral on gentle grades. In NACFE's 2019 fleet fuel study, fleets on average specified that shift-to-neutral feature on about 70% of the new trucks they bought in 2018. That's the transmission's software choosing a gentle grade, not a driver pulling it out of gear on a hill.

Watching the road far ahead saves fuel too. Cummins' list is plain: anticipate changes in traffic, avoid abrupt stops and quick changes in speed, and plan ahead so you use the service brakes less. Every time you brake, the fuel you burned to get up to that speed goes into heat in the brakes.

Tires: pressure first, then the right tire

EPA SmartWay says truck tires running 10 psi below the recommended pressure cut fuel economy by 0.5 to 1%, and that a low trailer tire can cost nearly twice as much as a low drive tire, because the trailer carries most of the load. NACFE adds that a tire running 10 psi low wears 5 to 12% faster. EPA notes that truck tires can lose up to 2 psi a month even with tight rim seals and valve stems. NRCan tells drivers to check pressure at least once a week.

Overinflating to cut rolling resistance is the wrong fix. Michelin warns that overinflation changes the tire's footprint and may contribute to irregular wear. Inflate to the tire maker's load and inflation table for the weight you carry, and check pressure cold, as Michelin says, before driving warms the tires.

If you're buying tires, EPA says SmartWay-verified low rolling resistance tires can cut fuel use by 3% or more on a long-haul tractor-trailer. That figure assumes verified tires on every axle position of the tractor and the trailer, all properly inflated, so a single set on the drives will save less.

The air: trailer gap, skirts and wind

Cummins' rule for aerodynamics is that every 2% less drag gives about 1% better fuel economy. A driver controls a few pieces of that. Natural Resources Canada says keeping the tractor-trailer gap to about 25 inches (64 cm) where possible can prevent a 3 to 5% increase in fuel use. Slide the fifth wheel only as far as your steer and drive axle weights, your turning clearance and your carrier's rules allow.

Trailer skirts and tails are the owner's call. SmartWay verifies aerodynamic devices for 53-foot van and refrigerated trailers in savings bands of 1 to 3.9%, 4 to 4.9%, and 5% and up. Before you buy one, look it up on SmartWay's verified list.

Wind changes the math at any speed, because drag depends on how fast the air hits the truck, not how fast the truck moves over the road. At 60 mph into a 10 mph headwind your drag is the same as at 70 on a calm day, about 36% more than at 60 in still air. Crosswinds hurt as well, because a tractor-trailer's drag climbs when the wind comes at an angle. Cummins estimates that every 10 mph of headwind or crosswind cuts mpg by nearly 13%. Slowing down a few mph into a strong headwind takes back part of that.

Cold adds drag too. Cummins says every 10 degree Fahrenheit drop in temperature increases aerodynamic drag by 2%, because cold air is denser.

Weight: the part you control is small

NACFE puts the savings at 0.5 to 0.6% of fuel for every 1,000 lb you take off the truck. Most of your weight is freight, and that isn't your call.

Running half tanks is the version drivers argue about. Diesel weighs about 7.1 lb a gallon, so 150 gallons less is about 1,065 lb, worth about half a percent. On a tankful of 150 gallons that's less than a gallon of fuel, about $5. Pay even 4 cents a gallon more at the next stop and it's gone. Half tanks earn their keep when you're scaling close to 80,000 lb and need those pounds for freight.

What doesn't work

Additives and gadgets

EPA says it does not endorse, approve or certify fuel additives, and registering an additive with EPA does not include any check of the maker's efficacy claims. Most devices EPA tested over the years had a neutral or negative effect on fuel economy or emissions, and for fuel line magnets and ionizers EPA found no substantive effect. That testing was mostly on cars, and most of it is decades old. For trucks, EPA's verification program for clean diesel technology excludes fuels and additives, and SmartWay, which does verify truck fuel-saving equipment, has no additive category. In NACFE's 2019 study of 16 large fleets, fuel additives had a 3% adoption rate, near the bottom of the 85 technologies it tracked. Winter anti-gel additives have a real job, keeping fuel flowing in the cold, but that job isn't mileage.

Drafting behind another truck

The savings people quote come from automated platooning tests, where trucks linked by radar and vehicle-to-vehicle radio with automatic braking ran 20 to 75 feet apart. In a 2014 study by the National Renewable Energy Laboratory (NREL), the trailing truck saved 2.8 to 9.7%. A driver without that equipment can't hold that gap safely. The CDL manual calls for at least one second for each 10 feet of vehicle length, plus one more second above 40 mph, which is about 8 seconds for a 70-foot rig. NREL's tests never went past 75 feet, and 8 seconds at 60 mph is about 700 feet, so the only way to get the savings from those tests is to tailgate. The following distance page has the rules by state.

Idling instead of restarting

Oak Ridge lists the idea that idling uses less fuel than shutting the engine off and restarting it among the idling myths in its 2016 review of manufacturer guidance.

Where you buy fuel, and IFTA

This one saves money, not fuel. Under IFTA each state taxes the fuel you burn there, figured from your miles in that state and your average mpg, and you get credit for the state fuel tax already paid at the pump. Buying in a low-tax state doesn't lower the total tax you owe. It moves the difference to your quarterly return.

To compare two truck stops in different states, subtract each state's IFTA diesel rate from the pump price. In California, IFTA credits back a fixed 97.9 cents for each tax-paid gallon bought there from July 1, 2026, according to the California Department of Tax and Fee Administration. That rate includes a part tied to the state's sales tax on diesel, but at today's prices the taxes in a California pump price add up to more than 97.9 cents, and the rest is a real cost.