2000 Pontiac Sunfire Car wobbles around 45-55mph.

Common causes are a broken belt in a tire, a bent wheel, a warped brake rotor, and worn spots inside an inner cv joint housing. The wobble will be in the steering wheel when it's caused by an inner cv joint housing, and it will only occur under load. A broken tire belt will cause a shimmy in the steering wheel that you can usually see when going very slowly, as in a parking lot.

The next step would be to measure the "lateral run-out" of the front wheels. That is done with a dial indicator that measures down to.001". If run-out is excessive, as in more than about.045", a bent wheel is a possibility, but first you would remove it and remeasure run-out on just the brake rotor. If any is found there, the new rotor would be the first suspect, but you also must check for any rust, scale, or other debris that got stuck between the rotor and hub, or between the rotor and wheel. That happens rather often, especially with cast aluminum wheels that are starting to corrode.

It's pretty common too for new rotors to be warped. A light machining will take care of that.