Flat Tires?

Those look really good.

I've flattened the hard vinyl tires before with a lot of success. I sanded, carved, or otherwise removed a section of the tire (tread and sidewall) enough that it looked mostly flat, then melted it with a candle and pushed it onto a flat surface. the result can be seen on my 1969 Charger in the link below my signature.

Al's got a good point about the suspension needing to be adjusted, it does take a little tweaking to get it to sit right.

I recall a tip in FSM or the sister pub Scale Auto a while back about an iron bought at a yard sale used to make the tire flat on one side. With practice you should be able to get a bulge too.
Good point about the suspension Al, unless this is for a dio then adjust the ground.

 

You will also have to work with the suspension to partly increase the weight on that shock while adjuisting the other shocks accordingly. Simply flattening one tire will leave that tire floating or its opposite off the ground.

I dunno if this would work, dont do autos much, but you could cut the back inside (facing the center of the car, away from the outside).  Cut that out in a square, put superglue on the inside of it and press it against a hard surface while on the wheel, that would be my guess if you had some scrap tires to try it on

This is my first time looking around on the autos forum so someone may want to comment on this suggestion.  Could you heat the tires up till the bottom kind of melts flat under the weight of the car?

You might try grinding on the inside of the sidewalls til they wouldn't support the weight of the model, or you might try to see if there are some resin flats around.  Don't know of any off the top of my head though.