The fuel efficiency of cars varies depending on factors such as engine size, fuel type, and driving conditions. However, here are the top 10 most fuel-efficient cars released in 2021 based on their EPA-estimated combined fuel economy (highway and city):

1. Hyundai Ioniq Blue – Hybrid (59 mpg)
2. Toyota Prius Eco – Hybrid (56 mpg)
3. Toyota Camry Hybrid LE – Hybrid (52 mpg)
4. Honda Insight – Hybrid (52 mpg)
5. Kia Niro – Hybrid (50 mpg)
6. Toyota Corolla Hybrid LE – Hybrid (52 mpg)
7. Hyundai Elantra Hybrid – Hybrid (50 mpg)
8. Ford Fusion Hybrid SE – Hybrid (42 mpg)
9. Honda Accord Hybrid – Hybrid (48 mpg)
10. Toyota Avalon Hybrid XLE – Hybrid (43 mpg)

Note: The rankings and fuel efficiency may vary based on different sources and models. It is recommended to check the EPA website or official manufacturer information for the most accurate and up-to-date details.