the clutch pedal is depressed full stroke to the floor for every gear shift, to ensure the gearbox is disengaged from the engine. On a semi-truck with constant-mesh transmission (non-synchronized), such as by the Eaton Roadranger series, not only is double-clutching required, but a clutch brake is required as well. The clutch brake stops the rotation of the gears and allows the truck to be put into gear without grinding when stationary. The clutch is pressed to the floor only to allow a smooth engagement of low gears when starting from a full stop; when the truck is moving, the clutch pedal is pressed only far enough to break torque for gear changes. Theoretically, semi-trucks could have diesel-electric transmission, as electric motors have better torque at 0 RPM than diesel engines, but this would increase the weight of the cabin itself, at the expense of the cargo weight. Lights An electrical connection is made between the tractor and the trailer through a cable often referred to as a pigtail. This cable is a bundle of wires in a single casing. Each wire controls one of the electrical circuits on the trailer, such as running lights, brake lights, turn signals, etc. A straight cable would break when the rig went around corners, so a coiled cable is used which retracts these coils when not under tension. It is these coils that cause the cable to look like a pigtail. In most countries, a trailer or semi-trailer must have minimum 2 rear lights (red) 2 stop lights (red) 2 turning lights; one for right and one for left, flashing (amber; red optional in North America. May be combined with a brake light in North America) 2 marking lights behind if wider than certain specifications (red; plus a group of 3 red lights in the middle in North America) 2 marking lights front if wider than the truck or wider than certain specifications (white; amber in North America) Wheels and tires Although dual wheels are the most common, use of two single, wider tires, known as super singles, on each axle is becoming popular among bulk cargo carriers and other weight-sensitive operators. With increased efforts to reduce greenhouse gas emissions, the use of the super-single tire is gaining popularity. There are several advantages to this configuration. The first of these is that super singles reduce fuel consumption. In 1999, tests on an oval