Gas prices crept back up over Memorial Day weekend, and if you're loading the family into an SUV for a two-week loop through national parks, the fuel line on your trip budget is the one that surprises people. Most of the advice floating around about stretching a tank is either outdated or flat wrong, and a few of the moves drivers swear by actually cost them money. The frustrating part is that the changes that genuinely move the needle are dull and easy, while the ones that feel clever rarely do much.
So before you tape up your grille or coast down every hill in neutral, here's what holds up.
The air conditioning debate is mostly settled
You've heard the old line: roll the windows down to save gas instead of running the AC. It made sense in 1995. On modern vehicles at highway speed, it's backward. Open windows at 65 mph create enough drag that the engine works harder than it would running the compressor, and the SAE and Consumers Union road tests from the last two decades keep landing on the same answer. Below about 40 mph, windows down is fine and even pleasant. Above that, close them and let the AC do its job.
There's a wrinkle worth naming. If your AC system is old, leaking refrigerant, or fighting a clogged cabin filter, it draws more power than it should, and at that point the math gets murkier. Most people never check the cabin filter, which on a lot of vehicles is a five-minute job behind the glovebox. A $15 filter is cheaper than a tank of premium, and a starved blower makes the whole system labor.
Speed is the lever nobody wants to pull
Here is the uncomfortable truth about highway fuel economy: the single biggest factor you control is your right foot. Aerodynamic drag climbs with the square of speed, so the jump from 65 to 75 mph costs far more than the jump from 55 to 65. The Department of Energy puts the rough penalty at roughly 15 to 20 percent worse mileage for every 10 mph you add above 50. On a 1,200-mile trip, that's real money, and it's the difference between three fill-ups and four.
I'm not going to pretend you'll drive 60 across Nevada. Nobody does, and on some interstates the flow of traffic makes it unsafe to crawl. But setting cruise control two or three mph below your usual pace, especially on long flat stretches, is the closest thing to free money this list offers. Cruise control matters here for a second reason: it smooths out the unconscious accelerator surging that eats fuel on rolling terrain.
The neutral-coasting myth
Shifting into neutral on a downhill to "save gas" was once a real trick with carbureted engines. It is now actively worse. Every fuel-injected vehicle built in the last 25 years cuts fuel to the injectors entirely when you lift off the throttle while in gear and rolling downhill. Drop it into neutral and the engine has to burn fuel just to idle. You're also giving up engine braking, which on a long mountain grade is a safety issue, not just an economy one. Leave it in gear.
Tires, weight, and the roof
Underinflated tires are the quiet fuel thief. They flex more, generate more rolling resistance, and run hotter, which on a 100-degree afternoon outside Phoenix is also how blowouts happen. Check pressure cold, before you've driven, and set it to the number on the doorjamb sticker, not the maximum stamped on the tire sidewall. Those are two different numbers and people mix them up constantly.
Then there's the roof. A loaded cargo box up top is the most expensive convenience on a road trip. The aerodynamic hit from a big rooftop carrier can knock 10 to 25 percent off your highway mileage depending on the box and the vehicle, and an empty crossbar rack you forgot to remove after ski season is still costing you a few percent for nothing. A few things actually worth doing:
- Pull the empty roof rack and crossbars if you're not using them this trip.
- Put weight inside the cabin or a hitch-mounted carrier before you reach for the roof box.
- Clear out the trunk junk — a couple hundred pounds of forgotten gear and a full cooler of sand from last summer add up, though weight matters far less than drag at highway speed.
- If you must use a roof box, a sleek aerodynamic one beats a brick-shaped bin, among other smaller fixes.
What's not worth your time
Premium gas, in a car that calls for regular, does nothing but enrich the station. Unless your owner's manual requires premium, the higher octane buys you no extra mileage and no extra power. Fuel-saving additives and magnets-on-the-fuel-line gadgets are worse — they're a tax on hope. And topping off the tank after the pump clicks doesn't gain you a free mile; it risks pushing raw gas into the evaporative system, which is the opposite of free.
The one piece of conventional wisdom that does survive scrutiny is the boring one: a car that's overdue for maintenance burns more. A dragging brake caliper, a failing oxygen sensor, or a check-engine light you've been ignoring since spring can quietly cost you double digits in fuel economy. Get the light read before you leave — most auto parts stores will scan the code for free.
The number that actually matters
If you do one thing from all of this, make it the speed. Everything else is worth a percent or three; backing off the throttle is worth ten or more on a long haul, and it costs you nothing but a few extra minutes spread across a day you're spending in the car anyway. Set the cruise, check your tires the morning you leave, and skip the premium. The savings show up at the third gas station, somewhere around the state line, when your tank is still reading higher than it has any right to.