I just reread this, and see that you are actually looking for distance not speed (although speed is linked to distance...)
I have a lot of experience with producing cars that go "the distance" as we run competitions for this specifically - however, our cars don't stay on the track after the hill and are allowed to "free drive" on linoleum, cement, hardwood, or tile - depending where we run this. So, I don't know how much help some of this will be.
A few things abut stuff that has already been said.
Those Matchbox wheels really do sound cool going down track... And you would think that because there is less surface area on the wheels there would be less friction to slow it down. But, we have found that these type of wheels never travel as far. I think the width on the "tread" in proportion to the wheel is too great, making it essentially a cog with large ruts that hit the ground in a flat manner - its hard to explain without drawing - rather than a tire's smooth motion. So essentially, if you blew up a matchbox car to actual life size, you would have a very bumpy ride, as it would feel like you were constantly hitting a flat tire, or a pothole every few degrees of rotation.
As for needing more tread to get grip - grip is only needed when something is driving the axle and tires. Since we are relying on gravity, I wonder if making the surface as smooth as possible would not be best? Also, the idea to round the wheels sounds like a good one. I did this when I was a scout for the pinewood derby also, it seemed to work. Although, we have always found that thin wheels do not spin as well as normal wheels - I think it has something to do with the way the axle is connected to the wheel. We find them to be neither fast or far ranging in distance. I cant remember the model, but there is a three wheel car that is made by hot wheels that gets alright speed and distance.
There is a picture of it on my website vracing.weebly.com/the-distance-event.html if you look at the third row down, second car from the left... that car does alright in both speed and distance and it runs on three wheels I believe. (EDIT, I just went back and looked - it actually has four wheels... so scratch that.)
I think it goes without saying that cars with straighter alignment, that don't bounce off walls will go the farthest as there would be less friction points.
We don't put anything on our cars' axles or wheels, so I don't have much input on that. IS there a post on how to use graphite on die cast axles?