I was reminded of this expression from American Jonathan Richman’s song Roadrunner when I heard it on the wireless recently. It seems to be a contraction of “faster than miles an hour” and it clearly means very fast despite the odd grammar. Is it known elsewhere to anyone?

As far as I’m concerned it’s just a song lyric.  If anyone actually said it, I’d assume they were quoting the song.

There is no “n.” I listened to the song again to make sure, and checked a lyric site, which indeed had the line as “Going faster miles an hour.” It is a straight-up substitution of “faster” for a numeral; it is poetic and funny and it rocks.