The manual gearstick in Spin Tires can be used to shift gears manually. But did you know there is a number of gears you can use? I did not until this mod appeared. It clearly display the number of gears available for you. And when you rightlick the mouse you force the acceleration which means you leave the truck crawling its way out of the thick mud while you go and grab a coffee.
Download and extract the zip. Insert the file into the Texures.zip file from Spin Tires. Do not extraxt that zip into a folder. Open the zip and paste the file to overwrite the original.