also gearing will make a difference, doing 186mph in 5th normally requires either very high revs or a longer ratio diff/5th gear.

If a cossie is producing 500bhp at 7200rpm, and for example you needed 8k to hit 186mph, your not gonna be producing 500bhp at that rpm hence its hard to hit that speed as you may only have 450bhp or less.

probably at 186mph in the m5 your still in its powerband as it has 7gears to use