Why is the Gear Vendors Overdrive so popular on Drag Week? Racers know it will live behind 4,000 hp, can be installed in a weekend, and doesn’t share fluid with your transmission so it won’t get contaminated when your automatic detonates.
To truly understand the function of the overdrive, we are first going to hit you with some basic drag racing concepts. On an ideal quarter-mile pass, you leave the line at torque peak, and shift through the gears at horsepower peak until you cross the finish line. The gear ratios in the transmission, rearend, and overall tire diameter determine the engine rpm as you pass through the speed trap. A higher overall numerical gear ratio raises rpm, and a lower numerical gear ratio lowers it. Simple, right?
As tires get wider, they also get taller, and as horsepower increases, peak power happens later in the powerband. All of this requires a higher numerical gear ratio in the rearend to get the engine speed up before the end of the quarter-mile and get the car off the line in the first 60 feet. If you can get through the traps without blowing up the engine, generally more tire and more gear is better.
The downside? A numerically higher gear ratio also raises the cruising rpm. For example, using the equation rpm = mph x gear ratio x 336 x 1.00/tire diameter, a Drag Week car traveling at 80 mph on the freeway with an aggressive gear ratio like 4.88:1 on a 33-inch tire spins the engine at nearly 4,000 rpm. If you’ve spent any time in a quick Drag Week car, you know that is a lot of noise, engine wear, and fuel consumption for a 300-mile trip to the next track.
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