I got 63 (on the gps) out of my 4 speed 2.25 petrol S3 on the M3 one morning. I was drafting a lorry for ages, pulled out of his slipstream and slingshotted past him, like a bloody NASCAR!
On a normal A road, I quite often see 55, but it's most comfortable at about 40 / 45 (my sphincter's...