Some six-speed manufacturers offer optional ratios for first gear and overdrive sixth gear. For example, you can often choose between a 3.24:1 and 2.94:1 first gear and either a 0.89:1, 0.86:1 or 0.80:1 overdrive sixth gear. Choose the first gear ratio based on your bike's weight and its primary drive ratio. With a heavier bike and/or lower ratio primary gearing (lower numerically), consider a higher ratio first gear (higher numerically). Remember that many fuel-injected big twins have lower primary gearing (1.54:1 versus 1.44:1) than carbureted models, so choose accordingly. For high-torque big-inch engines, you may already have more low-speed torque than you really need, so a lower ratio first gear (lower numerically) may be in order. Additionally, a big motor can pull a taller sixth gear, so don't choose the overdrive sixth gear ratio (i.e., 0.89:1, 0.86:1 or 0.80:1) without first considering the displacement and power of your engine and highway riding style.
For four-speed Shovelhead owners looking for an extra gear or two, Baker Drivetrain offers a six-into-four gearset. It is a six-speed overdrive with a 1:1 fifth gear and a 0.86:1 sixth gear. The OD sixth gear is good for a 500-rpm reduction in cruising rpm at highway speeds.
Remember that six-speed transmissions are available as complete transmissions or gearset kits. Of course, a complete transmission is more costly, but then again you have a used transmission to sell, which can offset some of the costs. Additionally, some aftermarket complete transmissions include upgraded features like improved shift mechanisms, offering smoother, more positive shift action and reduced component wear. Such features may be worth the additional costs.
Installation Considerations
Installation of a six-speed tranny isn't a difficult procedure, but you may have to do a bit of clearancing and grinding to get everything to fit. Don't forget that you'll need a larger engine sprocket and longer primary chain to overdrive the primary when installing a direct drive six-speed. The good news is that these items are usually included with direct drive kits. Another consideration is speedometer accuracy after upgrading to a six-speed. You may need a speedo recalibration box if your bike uses an electronic speedometer.
Final Thoughts
Adding a six-speed tranny, whether it is a direct drive or overdrive, reduces rpm between 11 percent and 20 percent, which makes a world of difference for highway and interstate riding. Reducing vibration makes long rides less tiring and much more enjoyable. Sixers also reduce engine wear and improve fuel mileage. And if you choose the correct first gear ratio, you can have crisper acceleration, too. Six-speeds don't come cheap, but they provide a win-win situation when selected properly and installed correctly. Moreover, your "behind" and your passenger will thank you handsomely for spending your hard-earned dollars wisely on the sixer of your choice.
Calculating Drive Ratios
The final drive ratio is a function of multiplying the primary drive ratio by the secondary drive ratio. The primary drive ratio is determined by dividing the number of teeth on the clutch sprocket by the number of teeth on the engine's compensating sprocket. The secondary drive ratio is calculated by dividing the number of teeth on the rear wheel sprocket by the number of teeth on the transmission sprocket. For example, assume we have the following: 24T engine sprocket, 37T clutch sprocket, 32T transmission sprocket and 70T rear wheel sprocket. The calculations look as follows:
37 24 = 1.54:1
Primary Ratio
70 32 = 2.19:1
secondary ratio
1.54 x 2.19 = 3.37
final drive ratio
Now, let's assume we install a six-speed overdrive with a 0.86:1 ratio to the above gearing combination. What would be the final drive ratio of the overdrive sixth gear?
3.37 x 0.86 = 2.90
overdrive sixth gear
final drive ratio