July 30, 2021

Linkage Mag

Geared for the Automotive Life

Adding Overdrive to American Muscle Just Got Easier

Image: Tremec

Everybody loves a V8 with a four on the floor. That row-your-own glory is great — that is, until you get out on the highway.

GM Muncies, Ford Toploaders, and Chrysler Hemi 4-speeds used a final 1:1 drive ratio back in the day, and with your standard muscle car performance gear ratios of the 1960s, that usually means 3k-ish cruising RPMs at today’s 60-mph-plus highway speeds. Oof. If you’re like me, that makes you think twice about taking your muscle car out on a longer drive.

So what to do if you really want to drive your muscle car? Well, you can live with that screaming V8’s belching heat and monstrous fuel bill. Or, you can change your rear end gear ratio to something more highway friendly, such as a 3.08, which unfortunately also means losing some of the instant fun of aggressive 3.73 or 4.11 gears.

Another option is to install some kind of overdrive — maybe a GearVendors unit — which mounts behind your transmission and splits each gear. But that requires fabrication, and it may not be an option depending on your car’s wheelbase and floor configuration. Same deal with the fantastic — but large — T56 Magnum 6-speed. They just don’t always fit easily, and that’s a problem for people who don’t want to go cutting up the floors of their muscle cars.

The T56 Magnum is a great transmission, but it’s too large for a lot of muscle car transmission humps, so custom fabrication is often required to make it fit. The TKX only has five speeds, but it’s smaller in size.

For those of you who want to keep your muscle car’s overall essence — and floor — intact while gaining some longer legs on the highway, there’s a new option for you: Tremec’s TKX 5-speed manual.

This transmission is an all-new design — meaning that while it shares some of its basic dimensions with the TKO 5-speed, it isn’t adapted from a factory-based unit, featuring a streamlined case for much easier installation in cars that had 4-speeds from new.

Three shifter locations make it easily adaptable, along with great additional support from companies like American Powertrain, which have crossmember and shifter options already worked out for most applications. Fourth is still 1:1, but the fifth gear ratio ranges from 0.72 to 0.68:1, depending on your preference. That will drop highway RPMs considerably, mellowing out that once screaming V8.

American Powertrain offers adjustable shifters and custom parts that adapt the Tremec line of transmissions to a wide variety of muscle cars. This is the White Lightning mid-shift kit for a T56 Magnum 6-speed.

If you’ve ever driven a modern Viper, Challenger, CTS-V, Camaro SS, or GT500 with the TR6060 6-speed manual, or anything with a T56 Magnum, then you already have an idea of how the shifts will feel in the TKX: bolt-action smooth and direct.

The nice thing here is that installation is much closer to a bolt-in affair than ever before — but you will need to have a different driveshaft made. Still, this is a reversible modification that won’t hurt your car’s originality — just save the factory parts.

Learn more about it at www.americanpowertrain.com and www.summitracing.com.