Kifli es levendula shows how to take a toilet paper tube, paint it and glue on some wheels and cute embellishments to make a toy car. Although the site is not in English it has lots of detailed photos.
Good for ages 7 and up.
Get the instructions here:
http://kiflieslevendula.blogspot.com/2011/07/kisautok.html
Mama jenn has a simpler model using bottle caps for wheels and straws to attach them to the tube.
Get the instructions here:
http://mamajenn.com/blog/2009/11/rollin-race-cars-tp-tube-style.html
Nerdy baby adds sails to their version and uses foam wheels with their toilet paper roll cars.
Get the instructions here:
http://nerdybaby.blogspot.com/2012/01/sail-cars-preschool-preview.html
Just be happy life adds cute little star stickers to their wheels.
Get the instructions here:
http://www.justbehappylife.com/2012/07/toilet-paper-tube-cars.html
Milenok makes a colorful version with large wheels.
Get the instructions here:
http://milenok.blogspot.com/2013/02/23.html
Tpcraft uses plastic bottle caps and straws.
Get the instructions here:
http://tpcraft.blogspot.ca/2011/02/toilet-paper-roll-race-car.html