Here’s some tips I use:
-put your axles in the dremel and smooth them with some 2000 grit sandpaper.
-put a Q-tip in your dremmel and ream out the inner wheel hub a few times on the highest speed to smooth the inside of the wheel hub
-place axles and wheels in a baggie filled with dry graphite and shake them occasionally for a few days prior to installation (after posishing the axles and inner wheels)
-PRESS the axles in with a clamp 9make sure to put a removable “c” shaped spacer between the wheel and chassis so you can remove the spacer once the wheels are pressed in and trued.
-check, double check, then recheck on a flat surface that all 4 of your wheels are touching the ground and level.
-On derby day, take a pocket of change and some super glue. you can always glue a coin or two on your car if you’re light on weight.
From a friend of mine on another forum.
Heres some other tips i gathered around for you.
Im so bored.
http://www.derbyworx.com/video.htm
the biggest speed secret is to make SURE your axles(little nails) are square or 90 degrees to the body.
The sandpaper the axles smooth seems to have worked for us so far, and yes the 90 degrees to the body is a MUST for speed.
Here’s my Axle/wheel tips:
I built a tool out of a c-clamp to press the axles in place rather than hammer them in, ensuring a true 90 degree to the body and ground and a 1/32nd space between the hub and body to avoid any excessive rubbing. 
Then before putting the axles in, I polished the hell out of them with some 200 grit paper on the insides of the hub and on the axle surface. Pretty much anywhere the wheel makes contact with the axle.
Take a Qtip in your dremel with some mild abrasive like toothpaste and use it to polish the inside of the hub where it rides on the axle. A little dab goes a long way! Be sure to remove all the polish before adding graphite. now you take and put your graphite into the hub and shake it with your finger over the holes, to ensure it’s coated the whole inside really well.
Now take your axle and carefully push it through the hub so you don’t knock out too much graphite. and press it onto the chassis.
Once all 4 axles are on, set he car upright on a true flat surface and check for any unevenness. if anything is uneven, take a small flat screwdriver and tap gently on the lowest axle to move it up in the axle slot to get the wheels all flat on the table. once you’ve done this, lay out a small puddle of water to roll your wheels slowly though. You’ll see on the wheels, what part is on the ground and what part is in the air. with 400 grit, gently smooth the area that will be contacting the track. I’m not saying reshape the wheel here, you just want to make sure the area on the ground is smooth and not the textured ridges on the outer edge of the wheel. Note: reshaping the wheel is illegal in most all scout races.
Once the tires are smooth and the axles are trued, it’s time to pour in a crap load of graphite. Turn the car over and lean it towards the corner you want graphite in. Tap your graphite onto the axle and axle groove so it makes a nice pile by the wheel hub. Now gently spin the wheel slowly by hand (not a fast free spin!) to get the graphite to fall into the hole around the axle and into the hub. (repeat for all 4 wheels/axles)
Now, give the wheels one good spinning to see how long they’ll spin. If you don’t get a 20 second spin from that wheel, graphite it again until you do. Repeat for all 4 wheels.
NOW, wrap the car in a paper town to keep keep any moisture away from the graphite and pine wood. Don’t take it out to show it off over and over, this spills the graphite every time you spin the wheels!
The day of the race, graphite the hell out of the axes again. I’ve noticed damn near everyone that handles the cars spins the wheels or runs it back and forth on their palms before putting it on the track to race.
the quotes are all the same guy, his sons took first and fourth out of 21 kids 
He put most of the weight over the rear wheels.