Love this subject but as always things are not what you think.


The Sierra left Ford with 205/50/15 tyres giving = 23.7mph/1000rpm
Every Tom, Dick & his Harry then fit :
225/40/17 = 24.75mph/1000rpm or 225/40/18 = 25.77mph/1000rpm if you change the Diff to 3.9 on the 225/40/18 = 23.92mph/1000rpm so you need the 3.9 to get it back to standard gearing.
The correct gearing is the shortest gearing that will give you grip no point fitting a 3.9 on mine it would spin its wheels at alltimes everywhere .
The shortest i can go is 3.54 diff with 235/40/18 = 25.72mph/1000rpm.

You can indeed reach 180mph at Brunters with a 3.9 provided the Tyres selected allow you to reach 180mph at roundabout max power.