There are more than enough lists of modern sports cars on sale today, the best used sports cars, cars with more than a certain horsepower output, etc. But what if you want a more subtle, lesser-known model? Modern sleepers might include the Lexus IS 500 with its 5.0-liter V8 and unassuming exterior styling. From an American perspective, there isn't much left aside from electric models like the now-discontinued Tesla Model S Plaid and ridiculously expensive Lucid Air Sapphire. Brands like Ford and Chevy abandoned their old sports-car lineups, except for one model each. For a proper American sleeper at a more reasonable price, buyers must turn to the used market.Finding underrated sleepers isn't as easy as buying an old sports car. Coupes and convertibles are too obvious, and SUVs haven't always had outrageous horsepower figures. That said, there are a handful of powerful American models that younger enthusiasts today might not know exist, and older enthusiasts may have forgotten entirely. Here are five of the most underrated used sleeper cars with American origins.Specs have been sourced directly from the manufacturers. Where 0–60 mph times and/or top speeds were not officially available, independent tests were used. This list is organized by model year from oldest to newest. Buick GNX Model Years: 1987 1987 Buick GNX front 3/4The oldest American sleeper car on our list is also the most expensive by far. The 1987 Buick GNX, or Grand National Experimental, is a rare, powerful two-door coupe with boxy, angular styling that took to the streets as a road-going variant of the manufacturer's NASCAR from the time. This model debuted during a lull in the performance car market. The era of classic muscle cars had died down after the early 1970s, and modern performance models hadn't taken off yet. That said, the automaker still decided to equip the GNX with a turbocharged engine that made an official 276 horsepower. Independent testing found that the model made closer to 300 hp, though.Forgotten by many, the GNX is certainly not undervalued today. Multiple sources confirm that only 547 units were produced, making this an exceptionally rare collector's item. It's a shame they don't get driven like they were built to anymore, but it's hard to fault collectors when mint-condition examples have sold for a quarter million dollars. Hagerty's valuation tool pegs perfect-condition examples at $267,000, while "good" condition models are still worth $125,000. Since they're so rare, we can expect those examples in good condition to hold their value. Ford BA Falcon XR6 Turbo Model Years: 2002–2005 2005 BA Ford Falcon XR6 TurboThough originally reserved for the Australian market, the BA Ford Falcon XR6 Turbo is built with American engineering. This is one of the most unassuming models on the list, since it looks like any other simple Ford sedan. However, beneath the skin of this boring-looking Ford is a Barra 4.0-liter turbocharged inline-six that makes 320 horsepower and 332 lb-ft of torque. That's enough to launch it to 60 mph from a standstill in about six seconds before reaching a limited top speed of 155 mph.We know what you're thinking: why would we include this model if it wasn't sold in the United States? Well, the fact that it wasn't sold here is exactly what makes the Falcon XR6 Turbo an exceptional sleeper. Nobody here would expect this little sedan to be so powerful.As for current values, it's difficult to put a number on it. Many of you are already aware that the United States requires cars to be at least 25 years old to be legally imported. That said, this version of the XR6 is only just about to become available to import into the US. Some listings for average-condition examples in Australia are priced between $12,500 and $16,000 AUD; American importers would then have to pay for shipping, customs, and other fees to legally register the sedan here. Pontiac G8 GXP Model Year: 2009 2009 Pontiac G8 GXPPontiac's final muscle car was the G8, with the GXP variant being the most powerful. While it was sad to see the brand go, it went off with a bang by offering one of the most underrated sedans of our time. The G8 GXP is powered by the same LS3 as the Chevrolet SS, which we'll talk about next, as well as the Holden Commodore it shares a platform with. The G8 is also worthwhile for mostly subtle styling that the average driver wouldn't expect. Even other enthusiasts would have to double-take to recognize it's not just any G8. Plus, these cars are still worth a decent chunk of change.These cars don't come cheap. The G8 GXP was only available for a single model year, 2009, and only 1,829 units were built. Keep that in mind when you see the average value still hovering around $41,000 today. If you manage to keep one in great condition, which may be difficult considering how fun they are to burn rubber in and toss around corners, this could be a future classic that gains value. Chevrolet SS Sedan Model Years: 2014–2017 2014 Chevrolet SSAnother lesser-known General Motors sleeper that often gets overlooked is the Chevrolet SS sedan. Chevy has used the SS, or Super Sport, badge for decades, and decided it was finally worthy of its own model for the 2014 model year. The SS sedan looks muscular to those who know what they're looking at, but the average driver wouldn't know it's powered by a 6.2-liter V8 — the same V8 found in the Corvette and higher-trim Camaros at the time.This Chevy is also an import in a way. While the Ford XR6 never officially reached the United States, the SS sedan began life in Australia as the Holden VF Commodore. While it was a Chevy version of the model, each unit came from Holden's Adelaide plant.Prices are still somewhat high for the Chevy SS. Yes, it's a relatively unassuming sleeper sedan, but enthusiasts in the know are still willing to pay over $40k for well-kept examples. The CarBuzz Marketplace Price Trends data shows the 2017 model year's average value is $44,600 today. The 2014 model year is slightly more reasonable at $33,900. The older models are long past the worst of their depreciation curves and could hold their value over $30k for a few more years. Lincoln MKZ 3.0T Model Years: 2017–2020 2017 Lincoln MKZ 3.0T FWDThe most recent American sleeper is one that sold right under our noses. Lincoln offered the 2017–2020 MKZ with your choice of either a standard 2.0-liter turbo inline-4 or a 3.0-liter twin-turbo V6. That's right: Ford took the current Bronco Raptor's powertrain and stuck it in a luxury sedan that most people wouldn't look twice at on the street. It's the perfect sleeper sedan for those who prefer a more comfortable, luxurious ride. Models with a front-wheel-drive configuration were tuned to 350 hp and 400 lb-ft of torque, while all-wheel-drive units were upgraded to 400 hp and the same torque.Values vary depending on the model year, image, and condition. That said, the MKZ follows a fairly typical depreciation curve for luxury sedans, despite its desirable engine. According to CarBuzz' Price Trends data, a 2020 MKZ Reserve with its 3.0-liter engine is worth about $21,000 today. That means it lost about half of its value over the last six years. We can see these sedans continuing to drop in value over the next few years, as well. Buyers looking for a luxurious American sleeper are in for a treat with the MKZ. Still, they should act now before collectors realize what they're missing and prices for rarer, more powerful examples rise again.Sources: Chevrolet, Lincoln, Ford, Buick, Pontiac