One of the favorite modifications for racing is changing to a higher gear ratio. I say for racing, because there is not power gain in changing to higher gears. In this example we have our 1997 Ford Mustang Cobra project car with a newly installed set of 4.30s. These gears really make this a fun car to drive as you are practically always in your power band. One of the drawbacks, however, is that your RPMs are always higher (so bye bye to fuel efficiency) and as a result your driveshaft is spinning faster at slower speeds. This situation leads to a vibration that is reported by many Mustang owners who switch to higher ratios. This vibration would still be noticed in the stock setup but you would have to be going much faster and sustain the speed to feel it. With the higher ratio you are sustaining the higher drive shaft speed at speeds that you often drive at. In this car at 3200+ rpm in 5th gear at 80 mph the vibration was pretty bad. In the stock setup, the speed needed for the same vibration is reported in excess of 100 mph.
So, what is the solution to this annoyance? Many people switch to aftermarket drive shafts. Any imperfection in the balancing of the drive shaft will lead to unwanted vibration. The aftermarket aluminum drive shafts are lighter and better balanced than the stock. Once the driveshaft went in the vibrations went away, at least in the range one often drives. Now the vibration comes back around 110+ mph and is about half of what it was before. An interesting side note is that the new 2003 Mustang Cobras use an aluminum drive shaft
Now, as with all modifications that are not cosmetic, we like to do before and after dyno runs to assess any gains or losses. There had been some theories that the lighter, more efficient shaft, could potentially lower the percentage of power lost through the drive train. The before and after runs showed no difference. The after was actually lower, but that was because the installation only took 15 minutes and the car was hot. In addition the MAC cold air induction that is in this car was also hot, which has given us problems in the past (see MAC induction article).
At least this is a fairly inexpensive modification and worth installing at the same time as the gears. You can typically get these aluminum shafts for about $150.