The wheels you get with your car kit are not completely round. It’s hard to tell with the naked eye, but there are high and low spots along the tread on most wheels. They roll just fine, but those high and low spots keep the car from rolling smoothly. It bumps up and down instead. And movement is the enemy of speed.
There’s two ways you can get round wheels. One is to turn them on a lathe. A lathe is a power tool that spins the wheel around and slowly shapes it until all the sides are the same. Not only are lathes expensive and hard to use, it’s against the rules in lots of races to modify your wheels that way.
Most rules do allow you to manually remove imperfections such as mold marks by lightly sanding or scraping the wheel. And there’s a tool that you can use to scrape off the high points on the wheel, including the mold marks.
The Pro Wheel Shaver has a hub to turn your wheel on and a blade you lock against the wheel. Using the included instructions, you first find the low spot on the wheel and lock the blade against the wheel at that point. Then you turn the wheel, allowing the blade to scrape off the high spots.