299792458 m/s299792458 m/sThe speed of light in a vacuum (denoted c) was standardized in 1983 as 299792458 meters per second. (See the BIPM's SI base units page.) Any further improvements to the accuracy of the measurement of the speed of light will change the meter, not the speed of light.
There are a few places on the web where the speed of light is stated erroneously as 299972458 meters per second - in fact, I was guilty of committing this error for several weeks on an early version of this page.