I wouldn't be surprised if cars have a peak efficiency in that range; car engines are often used far outside their peak operating parameters, however.
Actually, I tend to think that a properly-designed electronic transmission could do a lot to improve automotive efficiency. My understanding is that engines operate by far most efficiently at wide-open-throttle; a throttle makes an engine run much less efficiently. Unfortunately, an engine which is running at pure wide open throttle has a very unstable output response: when the engine is producing surplus torque, it will speed up and produce even more surplus torque; when it is producing insufficient torque for the load, it will slow down and produce even less torque.
It would seem that suitable control electronics, however, could tame this behavior. If the electronics were to monitor the power being produced by the engine and the power demanded by the wheels to produce the desired behavior (maintaining speed, accellerating, or deccelerating at a rate insufficient to require braking), it could adjust the simulated "gear ratio" appropriately. To yield good performance it may be necessary to make gear-ratio adjustments hundreds of times per second, but with an electronic transmission that should not be a problem.