Transmission Gearing Explained

From time to time, people ask about the different manual transmission gearing options we offer here at LMR.com. Whether it is a Tremec T56 Magnum 6-speed or a TKX 5-speed, there can be a few different options to choose from. This knowledge of the different gearing can help you with the overall performance of your build. Pairing the transmission gearing and rear-end gear ratios can improve driveability in stop-and-go traffic and create desirable cruising RPM and highway speeds.

Transmission Gearing (1st Gear)

Let's use the S197 GT500 for an example here, the 2012 GT500 came equipped with a 3.73 rear-end gear, and the TR-6060's first gear was 2.97. This improved "off-the-line" performance, but it limited top speed overall. The 2013 GT500 swapped over to a 2.66 first gear, and a 3.31 rear-end gear because Ford wanted this vehicle to reach over 200 MPH. Along with a bump in power, this gearing helped them achieve what had never been done before in a factory Mustang. This pairing allowed 2013 GT500s to reach close to highway speeds in first gear alone.

Typically, a good rule of thumb here is the lower numerical first gear you have; it will allow you to stretch out the top speed before reaching the redline. This style pairing is normally found with matching lower numerical rear-end gear ratios. This will provide you with a reduction in overall RPM and cruising speeds and will help you gain an overall top speed.

Below speed is based on a 25.7" diameter tire with 6000 RPM redline, which will be similar to a 1979-95 Mustang 5.0L engine. Increasing RPM shift point or increasing the diameter of the wheel will net a high speed in each gear. Lowering RPM shift point or tire diameter will decrease the top speed of each gear. 

Overdrive Gearing

Since we have covered how the first gear and the rear gear coincide, we still have to figure out your overdrive ratio and how this comes into play. For example, most 5 and 6-speeds used in the Mustang feature a true 1:1 or direct drive in 4th gear. If you have a steep "off-the-line" rear-end gear like a 4.30, you would want an overdrive gear to help you provide the best overall reduction in RPM at cruising speeds. In comparison, if you have this 4.30 rear-end gear and choose a .82 overdrive, this is not far off from your 1:1 4th gear ratio. This will cause you to have higher RPMs than an overdrive like a .68 gear. In turn, if you have more highway-oriented rear-end gear like a 3.08, this .82 can benefit you by helping your engine stay in the proper RPM range it needs at highway speeds.

*This chart is theoretical and is not the actual top speed of your vehicle.