They’re not inefficient, they just have maintenance issues. If you’re willing to maintain the engine, you’ll be just fine. And because the Wankel is lighter than an equivalent piston engine, you save weight and therefore fuel.
A 2400 pound car* with 100 horsepower that gets well under 20 mpg is not efficient. My 2011 Ford F150 weighs 5,500 pounds, has 360 HP and gets better mileage. Not near as much fun to drive, though.
Check the specs of the 2011 Mazda RX8 and you’ll see the rotary engine is much less efficient than a turbocharged V6.
*Edit: I was referring to my 1980 RX7. The 2011 RX8 is heavier, faster and just as thirsty. According to fueleconomy.gov the 2011 RX8 is rated at 16 mpg city and 22 mpg hwy, exactly the same as my F150. However, users are reporting an average of 14.9 mpg. The F150 has a user report of 15.8 overall. I’m averaging 17.2 overall. Take these with a grain of salt, though, as both the RX8 and F150 have only one user report, and we do not know their driving habits. I suspect they are both reporting urban driving. I have gotten 24 mpg highway with my F150 driving 70 mph on I-35.
Edmunds.com lists the RX8’s weight as 3111 pounds, and the 1.3L engine yielding 212 HP and and 159 lb/ft torque. My F150 has a twin-turbocharged 3.5L V6 with 360 HP and 420 lb/ft torque, yet gets essentially the same mileage as the RX8. Just because you save weight with a Wankel does not mean you will save fuel. That was the thinking back in the 1970s when car companies started experimenting with the design. All but Mazda realized early on that the rotary engine was not a viable alternative to reciprocating piston engines. Newer does not always equal better.