Get in the Grind with a Harley-Davidson Six-Speed Transmission

Transmission Solution

Riders of older Harley-Davidson are sometimes left hankering for one more up-shift. While five-speed transmissions work just fine, six gears has its benefits. The solution is either a complete transmission swap or a new gearset installed into the existing box. There are also two kinds of transmissions available for Big Twins — overdrive and direct drive. Here is an overview. 

Slow Shift

Harley-Davidson makes slow shifts when it comes to updating its transmissions. The four-speed gearbox was introduced in 1936 along with the new EL 'Knucklehead.' The Motor Company employed versions of the four-speed for almost fifty years, until the introduction of the five-speed in 1980 on the FLT Tour Glide. It then took half as many years for the six-speed transmission to be introduced in 2006. 

Photo courtesy of Mecum Auctions.

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Gear Advantage

The main advantage of a six-speed transmission is lower engine speed and reduced vibrations in top gear. For those who put down serious highway miles, this is a real consideration. While it is possible to raise final gearing via the pulleys or sprockets, this also raises the lower gears resulting in reduced acceleration especially riding two-up, with a load or climbing hills. 

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Direct Overdrive

Harley-Davidsons can be fitted with either an overdrive or a direct drive transmission. Overdrive transmissions are nothing new in the automotive world, but they are relatively recent on Harley-Davidsons. Starting with 2006 Dynas, and 2007 and up Touring models, the six-speed Cruise Drive transmission was fitted as standard equipment. Interestingly enough, the Cruise Drive transmission is a direct drive and not an overdrive design. 

Photo courtesy of Jalopnik.

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Performance Priorities

To be classified as an overdrive, a transmission must have at least one gear with an internal ratio of less than 1:1. A common six-speed overdrive transmission will have a sixth gear ratio of 0.86:1. The same can apply to five-speed transmissions. A direct drive has a 1:1 internal ratio high gear, meaning the rear wheel turns one revolution for each revolution of the engine. 

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Direct Upgrade

There is an advantage to each transmission and it all depends on the application. It may seem counter-intuitive, but the direct drive is designed for high-miler touring bikes and the overdrive for performance-built machines. The Screamin' Eagle six-speed transmission and six-speed gearsets sold for upgrading 2006 and earlier five-speed transmissions are a true overdrive and not direct drive. 

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Performance Option

While a six-speed overdrive drops engine rpm and leaches a little bit of power, there are more gear options. Overdrive ratios can be tailored to the rider's style and engine output. By contrast, a direct drive transmits all the engine output to the rear wheel. A six-speed direct drive also has the benefit of more evenly spaced gears, which makes for smoother shifting, but the final gear is fixed at 1:1. 

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Six-Speed Swap

Baker Drivetrain, an authority in transmission issues, makes a valid point. They state, “To determine which six-speed is right for your application, you must look at your engine output, riding style, bike gearing, GVW, and performance priorities.” The other critical advice found on many HD Forum posts is to either work with a certified Harley-Davidson dealer or known aftermarket manufacturers when considering a six-speed swap. 

>>Join the conversation about transmission swaps right here the Harley-Davidson Forum!

For help with the service of your bike, check out the how-to section of HDForums.com.