Americans are now paying $4 a gallon for gas. See the map. Business Insider tells the global tech, finance, stock market, media, economy, lifestyle, real estate, AI and innovative stories you want to know. Fueling up just got even more expensive.On March 31, the national average price for a gallon of regular gas climbed to $4.018, according to AAA, crossing a key threshold that tends to grab drivers' attention — and strain household budgets. It had been $3.99 on March 30.AAA data shows the national average stood at $2.98 a gallon on February 27, a day before the US and Israel bombed Iran.The gas price run-up has accelerated amid the heightened tensions in the Middle East. Shipping through the Strait of Hormuz has been disrupted, where an estimated 20% of the world's oil supply flows. Meanwhile, ongoing attacks on regional production facilities have choked supply.Those factors have pushed oil prices to multi-year highs. Brent crude futures, the global benchmark, reached $117 in trading on Monday, March 30. The last time the prices spiked above $100 was in the summer of 2022.In the US, higher gas prices have been particularly pronounced in the West, where a string of states from Washington to Arizona have been paying north of $4 a gallon since March 12.At an average cost of $5.89, California has the most expensive fuel as of March 31, while Oklahoma has the cheapest, with a gallon averaging $3.27.Seasonal dynamics are also working against American consumers. Spring marks the transition to summer-grade gasoline, which is more expensive to produce, as demand ramps up ahead of the busy driving season.Carol Spieckerman, a retail analyst, told Business Insider that the impact of higher gas prices is "far-reaching and underestimated" for the entire US economy. She said that higher fuel costs have led to higher prices for airline tickets, groceries, and petroleum-based products — and those higher prices are harder for small businesses to weather."If you're a specialty retailer in a single market, higher gas prices mean fewer shopping trips and more cautious spending when customers do show up," she said. "By comparison, retailers like Walmart that serve as convenient one-stop shops and offer delivery in many areas will benefit. Amazon, which requires no travel at all, obviously gains ground."Still, there are potential off-ramps, particularly if tensions ease, production rises, or governments tap strategic reserves.For now, Americans are once again confronting a familiar reality: higher costs everywhere — from the checkout line to the gas pump.