Americans appear to be gearing up for further increases in US gas prices, as green-car sales last month had their largest year-over-year gains of 2014. Domestic customers bought almost 56,000 hybrids, plug-ins and diesels in April, marking a 2.6 percent increase from April 2013. Plug-in sales were particularly strong, jumping 41 percent from a year earlier, as sales of the Chevrolet Volt extended-range plug-in and Nissan Leaf and Tesla Model S battery-electric vehicles all showed gains.

April's big winner among the automakers was Honda, moving 1,442 units of its newer Accord Hybrid

And April's big winner among the automakers was...Honda? Yes, Honda, which has long operated in the advanced-powertrain shadow of fellow Japanese automakers Toyota and Nissan, came up big by moving 1,442 units of its newer Accord Hybrid. And while sales of the Civic Hybrid, CR-Z and Insight all fell, the Accord Hybrid drove Honda to boost its green-car sales by 78 percent from a year earlier to 2,839 units.

Per usual, Nissan and Tesla also showed year-over-year gains. Nissan boosted Leaf sales by 7.8 percent to 2,088 units. And while Tesla won't release its first-quarter results until May 7, the California-based automaker would've increased Model S sales by 34 percent to 2,300 just by maintaining its fourth-quarter 2013 sales pace.

Volkswagen and low-volume advanced-powertrain vehicle makers like Audi, Porsche and Daimler AG's Smart division also fared well in April. VW increased its diesel and Jetta Hybrid sales by 25 percent to 9,583 units. Audi's diesel sales quadrupled to 2,088 units. Smart sold 203 units of its newer Smart ED battery-electric vehicle.

Such gains more than offset sales declines from General Motors, Ford and Toyota, though Toyota's April was less painful than previous months.

GM's big mild-hybrid sales declines more than offset the 19 percent increase in Chevy Volt sales to 1,548 units and the sales of 491 Chevrolet Cruze Diesel vehicles. All told, GM's green-car sales declined 25 percent to 3,103 units.

Fusion Energi Plug-in Hybrid sales doubled and C-Max Energi PHEV sales jumped 28 percent.

Ford's green-car sales were down 12 percent to 7,554 vehicles. While Fusion Energi Plug-in Hybrid sales doubled and C-Max Energi PHEV sales jumped 28 percent, Fusion Hybrid sales were little-changed while C-Max Hybrid sales tumbled 50 percent to 1,586 units.

And Toyota, whose first-quarter green-car sales were down 21 percent, saw its April sales fall 5.7 percent from a year earlier to 26,663 vehicles. Declines in Prius liftback sales and Lexus hybrid sales more than offset the effect of an increase in Prius Plug-in Hybrid and Avalon Hybrid sales.

US gas prices are already above last summer's highs.

US gas prices are already above last summer's highs and have jumped about 40 cents since the beginning of the year to about $3.70 a gallon, according to AAA. Check out our comprehensive sales chart by scrolling below.