Just jack up the rear and see how freely the wheels rotate. Calipers could be dragging due to a collapsed flex line.
Nick

 

Old (even with tread) or worn tires will make you uneasy in a hard corner. Just bought a z31 with old tires. She goes out easy on a corner. Especially these last few days. I'm in Mill Creek. Did you replace ALL the bushings or just those you mentioned? New ball joints should have been in the mix for what else you did. My 83 used to wander the road til I did urethane bushings, ball joints and tie rod ends. Made things considerably tighter. n/a s130 isn't the quickest thing on the road but i doubt wheel bearings would cause a lot of drag. More likely a brake dragging but you should feel a lot of heat if you drove down a street a mile or so then pull over. Coast to a stop minimal braking then jump out and feel the rotors (if they are blue that is a dead giveaway) My left rear went out but I could hear the noise. Check out our club. zccw.org we are always willing to give advice and meet new people. We have a club activity every month on our calendar.

 

Make sure your parking brake cable system is not rusted or damaged, causing the brakes to drag a little. You said your calipers are new. Try disconnecting the brake cable from both calipers for a few days and see if there is a difference. Just be sure to leave the tranny in reverse when parking, crabbing the front tires into the curb, et cetera, to safely make up for the lack of parking brake while making the experiment.

 

RW Bearing Drag

If the rear wheel bearings are dragging enough to affect fuel mileage, take it from me, you WILL hear them!

BRRRRRRPRRRRRRPRRRRRRPRRRRRRPRRRRRRP, with an increasing level as you corner and put more load on the bad bearing.

#1 Problem with 'professional' rebuilds on ZX Rear Brakes these days is techs doing the work that are younger than the car and are under the mistaken impression since they are 'calipers' they will 'free float' and there is nothing to do to them but put pads in... This goes for NEW calipers as well: You have to make sure the calipers are SCREWED back into the caliper body before installing the shoes. This is their self-adjustment procedure. They have to be ALL THE WAY SCREWED IN to the bodies, and then the first time you apply the service brake, they will come out and self-adjust. E-Brake Cable adjustment shold be checked as well, but if you're smelling it chances are they are cooked and you need new pads at least.

Take a look and let us know what you find...

 

I know what you mean about it feeling heavy, that's why I stripped mine, but I wouldn't recommend that to anyone. Check for free rotation of the wheels when jacked up, as others have said. Does it smell like brakes? Mine was very hot and smelly, but only by the driver's rear tire. I assume it was the heat from the catalytic converter melting stuff, since the problem mysteriously disappeared after the appropriate modifications.

And if you're riding on old tires, replace them. They're the difference between losing it on an entrance ramp and sticking no matter how hard you push.