Need Advice
 

So when I bought my car the owner put on some new tires but im positive they are not the right size. 245/45/17 rear 245/40/17 front....my question is how is this affecting my car (spedometer, performance, oversteer/understeer etc etc..) Also, if performance is my main concern what size should i get? i have an 05 on stock wheels and stock suspension.

215/45, 245/40 is stock sizing to get an idea.

noob here so :ponder: explain please

A couple of issues here. You have a 245/40 on the front 7" wheel. Most tire manufacturers recommend running at least a 8" wide wheel with a 245 tire.

You have a 245/45 in the rear. This diameter is oversized by 1" compared to the stock 245/40. Your speedometer will be off, reading slower than your actual speed.

You are running 245 width tires all around. This will really increase oversteer. A lot of track guys do this, but usually counter it with a stiffer front sway bar and suspension tuning.

I would get a matching pair of 215/45 for the front and move the 245/40 to the rear. You will need to have the 245/40 dismounted and moved to the 8.5" rear wheels because I am assuming they put them on the 7" front wheels.

You can also upsize to 225/45, 255/40 but I would wait since you already have a perfectly good pair of 245/40 for the rear.

if i didnt change anything and just waited till they wore...would the stock size be the best for handling with the stock suspension when i do get new tires? also what i have right now wont hurt the car right?

The OEM tires provide a very good driving experience. If I were you I'd start there and try different tires sometime later, after you have a basis to compare.