What you're experiencing is a swing from one extreme to the other. Coming from an AWD subaru that rarely loose their composure in any weather, to a powerful rear drive manual car with a solid axle, locking rear diff, and brakes biased to the front is going to be a huge learning curve......be it a mustang or something else. Contrary to whats said above, the v6 mustang is pretty well sorted weight distribution wise for a front engine car, 54/46 if I remember right.......so that's not the culprit.
A few things you're going to have to accept with the mustang that are just par for the course.....
1. do not downshift unless absolutely required.......and even then don't make the move until you match the gear you're going to by 10. As in, don't downshift to 3rd until you are at roughly 30mph.
2. limit yourself to no more than half throttle unless driving perfectly straight in rain
3. likewise try to do the majority of your braking in rain while perfectly straight.
4. if you know you're about to go through standing water at speed......feet off the pedals, both hands on the wheel.
These things should keep you safe until your fully adjusted or at least more familiar.
Also, IMO the nero's are just a crap design as for water evacuation. Opposed to a directional or asymmetrical tire that evacuates water in one direction, or across the tire in the direction of travel. The nero has water channels on both sides and opposing directions on either side. I would think that the tire fights it's own momentum to evacuate water because of this.....at least standing water.
2012 Kona Blue 3.7 MT82
Airaid CAI, Flowmaster A-back, BBK ceramic shorties, black enkei raijin 18x8.5, Steeda ultralites/pro action dampers, whiteline PHB & brace
RJW Aggressor matte black stripes, GT bumper/CS valance/Saleen grille, AM black shift knob
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