Did you know that the TREMEC Magnum 6-speed is available with optional gear ratio sets? This is one of the things that makes the Magnum even more versatile for retrofits into muscle cars, hot rods and a variety of competition vehicles.
It helps to understand transmission gear ratios, how they affect rearend axle gear ratios and why it’s so important to match these transmission and axle ratios properly. Transmission gear ratios are designed to work with a range of rearend gear ratios so that the engine operates within the part of its power band that generates the most power. To select the ideal gear ratio, you must take into account vehicle weight, engine camshaft specs, the powerband, optimum engine rpm, intended vehicle use and the axle gear ratio. Meanwhile, transmission overdrive ratios are designed to work with rearend gear ratios to put the engine in an optimal lower-rpm range for highway-speed cruising and fuel efficiency.
When the TREMEC Magnum 6-speed entered the performance world in 2009, it changed the game, offering the capability to handle big power from modern performance engines, high rpm shifting, fitment into a variety of vehicles/applications and optional gear ratio sets to further expand the applications and scenarios in which it could be used.
With additional R&D and real-world testing, and listening to input from our distributors and customers, TREMEC has decided to eliminate the .74/.50 overdrive gear set on wide-ratio variants of the Magnum and Magnum XL (versions with 2.97 first gear ratio) and replace them with the preferred overdrive gear set of 0.80/0.63. The wide-ratio first through fourth gear sets will remain unchanged (2.97 first gear, 2.10 second, 1.46 third, 1.00 fourth).
Using the TREMEC Gear Ratio Calculator, you can see what combining the shallower overdrive ratios with the wide ratio gear sets will do.
For our purposes, we’ll use a 26.5-inch-tall tire with a 4.11 rear gear ratio, and a goal of 2,600 cruising rpm. Using the Gear Ratio Calculator, we entered the necessary data, and got our results. Our theoretical vehicle would be going 67.4 mph in fifth gear. Pretty much at the speed limit. To maintain 2,600 rpm and use sixth gear with the Magnum wide-ratio variant and .74/.50 overdrive gear set, we’d have to be traveling at 99.75 mph, essentially hitting the century mark. Bottom line, unless we’re on the Bonneville Salt Flats or a highway with no speed limit, sixth gear and its .50 overdrive gear ratio are not optional for normal highway cruising.
However, using our same vehicle parameters, but with the .80/.63 overdrive set, we see a much better result. In fifth gear and 2,600 rpm, our cruise speed would be 62.34 mph, and in sixth gear, it’s 79.16 mph, which is much more in line with normal highway speeds while keeping the engine in the sweet spot of its rpm power range.
If you want to see what kind of difference having the shallower overdrive gear ratios will make in your vehicle with a Magnum with the wide ratio gear set, you can use the TREMEC Gear Ratio Calculator to find out. And if it’s a change you want to make, call the TREMEC tech line at 800-401-9866, email us at firstname.lastname@example.org, or use our contact page to talk with a TREMEC specialist about what it takes to change out your overdrive gears.