I've fitted a 3.9 to mine (2wd) It deffo makes pottering about and pulling away with an evil clutch nicer than before, the whole car just seems more driver friendly.
Can't comment on performance as haven't had my car on boost much since running it
Originally Posted by
MadRod
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
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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.
Hi Rod,
Makes a lot of sense
How would my actual car speed compare to my indicated speed then? I'm runnning a 3.9 diff (2wd) with 235/40/17.
I guess not *too* far out at legal speeds?