Six-Speed Transmissions

Six-Speed Transmissions
Regardless of whether you're cruising the local freeways or jamming on the intestates to a major cycle event, you've probably found yourself wishing for another gear, wanting to upshift into a higher cog to reduce the relentless vibrations the V-twin engine emits at highway speeds. Raising your bike's final gearing would move you in the direction of having a higher gear, but it would also render all lower gears equally higher, reducing acceleration and making passing on hills when fully loaded more difficult. However, if you own a 2006 Dyna or a 2007 and later big twin, you found that extra gear without encountering the lethargic acceleration pitfalls: It's called sixth gear. Adding a sixth gear reduces vibration on the highway, making long rides less tiring and more enjoyable.

If your bike doesn't already have a six-speed transmission, there are two ways to upgrade: You can either replace your current tranny with a complete, new six-speed or install a six-speed gearset into a conventional five-speed box. Harley-Davidson and several aftermarket companies offer six-speed options for the big twin in both complete transmissions and internal gearsets. More importantly, though, the sixers are available in two critically different designs: overdrive sixth gear (OD) and direct drive sixth gear (DD). Let's take a look at both designs.

Overdrives
Overdrive transmissions are nothing new to the automotive world. In fact, you probably have one in your late-model grocery-getter. However, overdrives are relatively new to the Harley world, first appearing about the mid-1990s. A big twin five-speed transmission or even a Shovelhead four-speed tranny has a 1:1 internal ratio high gear. That means the rear wheel turns one revolution for each revolution of the engine. To be classified as an overdrive, a transmission must have at least one gear with an internal ratio of less than 1:1. An overdrive six-speed trans adds an overdrive sixth gear to a five-speed box. The OD sixth gear will have less than a 1:1 internal ratio, thus reducing engine rpm.

For example, big twin six-speed overdrive transmissions usually have an OD ratio of 0.80:1, 0.86:1 or 0.89:1, thereby reducing fifth-gear rpm between 20 percent and 11 percent. Although six-speed ODs are available in several different internal ratios, a 0.86:1 internal ratio, which decreases a standard transmission's fifth gear ratio (and engine rpm) by 14 percent, is a common choice. Here's another way of looking at it: Installing a 0.86:1 overdrive sixth gear in a trans with a 1:1 ratio/3.15:1 final drive results in fifth gear remaining with 1:1 and 3.15:1 ratios, but sixth gear becomes a 2.71:1 final drive ratio (3.15 x 0.86 = 2.71). By the way, I might add that overdrives are not limited to only six-speed transmissions. Five-speed transmissions can also have an overdrive gear where fourth gear is a 1:1 ratio and fifth gear uses a less than 1:1 overdrive ratio.

So you might ask, what does this mean to you when you are cruising down the highway? Depending on the year and model of big twin, the engine is typically turning between 3000 and 3500 rpm at highway speeds. If we assume your engine is turning 3200 rpm at highway cruise with a five-speed 1:1 ratio nonoverdrive tranny, installing a 0.86:1 ratio overdrive reduces engine rpm by 14 percent, resulting in a reduction of 448 rpm or 2752 rpm at cruise speed. That is a significant reduction in rpm. It not only results in happier cruising and reduced vibration but also reduces engine wear while increasing fuel economy.