1996 honda accord ex new 215/40/17 rims and lockers Saturday. I drove the car today about 10-20miles both freeway and city and I notice that on hard corners my rear will start to kick out like it's got nothing holding it tires aren't slipping as far as I can tell but rear definitely feels loose like it's being pushed sideways speed that usually occurs is 70ishmph before new wheels car felt planted up to 95mph stock suspension stock ride height

 

first thing I'd say is have the alignment checked out. beyond that, you may have bought crap tires

 

Some of this may be the dynamics of the car. You have larger wheels now, unless you got pricey forged wheels I will assume your wheels are now heavier, and your tires definatly are heavier. I also bet they stick out farther. This does give more grip, tho typically on our Hondas we get understeer.

Check the tires, make sure if they are "directional" tires they werent installed backwards on any of the tires. they usually have an arrow of some sort on the sidewall, and of course would then rotate 'front to back' only. Next is your suspension. Check your bushings to see if any of them are ripped and thus have a lot of play (slop, wobbling side to side, etc)

Also if you got your tires somewhere akin to like Kauffman, they are notorious for overinflating their tires. They set mine at 38psi (96 civic) when I was told with my old cast-aluminum wheels that they could bend on a pothole with the thin 40-profile tires I had. For years after that I ran em myself at 32psi, rode and gripped better.

Last thought. Your rear suspension may be softer than the front, as by design of the accord, its supposed to be somewhat more posh. This means at high speeds, where your old tires were fatter, by that i mean higher profile (195-65 or something?) and so would roll a little bit, onto the side of the tire and grip at those higher speeds. Now with your suspension deflecting relatively the same as it wasnt changed (but the tire side, track and weight altering it), your suspension may flex the wheel onto its edge, and your tires probably have less flex, meaning you're getting less tread on the ground.

Any other factor (real high tire pressure, busted suspension components or bushings) could add to this. So ultimately if you checked all I told ya, PSI is good, etc, car is in normal running shape, then lastly I'd have to say given the change due to the wheels/tires you may wanna look into a firmer front swaybar, springs, shocks, etc, all that stuff to help it corner at high speeds.

Hope this helps a bit.