ok so you want part of the model smoothed whilst the rest looking blocky... sorta.

this is assuming that the model is a complete set of gemeotry sharing the same transform node.

theres two ways to approach this:

short way:
i would do a part smooth operation, meaning select the faces on the tires (or areas you want actually smoothed) and then smooth it under polygons.
go into the attribute editor, locate the smooth operation tab (polySmoothFaceX) and under continuity reduce it to around 0.15 (play around with tab to get the effect of smoothness your after)

you can do this with the rest of the model too.

the end result, you'll have a fairly nice smooth of tires with the main body keeping its original shape but looking more.. intuitive (cant think of a word right now lol)

longer way:
convert to sub-divs and shape the form, keep switching back between poly and sub divs to get the shape your looking for.

(keep an eye on poly count)

Edit: grammar corrections