We mentioned how to save fuel economy be reducing overall vehicle weight in the weight reduction section. However we will now focus on something a little different. Driveline weight. Reducing driveline weight has an added advantage to just reducing vehicle weight and as such deserves a section of its own. Driveline components such as the driveshaft, axles, flywheel, crankshaft, etc. rotate and transfer power to the wheels of the vehicle in the process. These rotating components have weight, and energy is required to get this weight spinning in the first place. The heavier the rotating component the more power required to get it moving. This means more fuel wasted in the process. By reducing weight in the driveline components you reduce the rotating mass and free up additional horsepower which goes directly to the drive wheels of the vehicle. This saves you mpg's by not wasting additional energy.
1. Motor Components
Many motor components can be lightened to increase efficiency. The crankshaft, pistons, rods, and flywheel are examples. Having a lighter flywheel installed on a manual car has the added benefit of greatly improving the acceleration of your vehicle as well.
Most stock vehicles come equipped with a heavy steel driveshaft. Lighter, aftermarket drive shafts include aluminum and carbon fiber units, carbon fiber being the lighter of the two. In addition to freeing up extra power and fuel they reduce vibration in the driveline and can create a smoother vehicle ride.
3. Lighter Wheels
Aluminum rims are a popular lightweight upgrade to many vehicles. They weigh less than their steel counterparts and reduce the rotating mass of all 4 tires. In fact many new cars often come with aluminum wheels from the factory these days.
We do not have any before and after fuel mileage testing with respect to installing lightened driveline components and thus cannot provide a number on possible gains in mpg's. So do your own research if you want hard numbers, as purchasing many of these components can be a costly affair and you may very well wind up spending much more for the parts than the gains you realize at the pump.