You need to tidy up the edge of the tyres so they look round or remove them completely and just use the rims.

There are lot of ways to do it, probably the easiest is to join the two wheel images into 1 layer then adjust the contrast and lightness to blen the colour into that of the wheels already i.e. lighten the images until the black turns to the grey that matches the existing tyres, or like I mentioned above if you remove the tyres you then wont need to. You could then just adjust the layer visabilty slightly.

Try that for starters and see how you get on..........