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All About Axle Gears

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Axle Gears vs. Vehicle Performance
If you continue to visualize the example of removing lug nuts with the long or short wrench, it will help you understand how to gear your car for quarter-mile acceleration. Anything you can do to reduce the resistance of the lug nut turning will allow you to either use a shorter wrench or less effort; similarly, the lighter the car, the less the effort required to move it, the higher the gears you can get away with. Also, your musclebound buddy can probably loosen a lug nut with a short wrench, whereas a weakling requires a longer wrench; it's the same on a car-the more input power (engine torque), the higher the gear you can use.

In the real world, typical street machines with aspirations for good dragstrip performance generally run quickest with 4.10:1 gears. Lower gears are required if the car is very heavy, or if the engine makes its power at the upper end of the rpm scale. Sometimes higher gears are used if the engine has gobs of low-end torque and doesn't like to spin at high rpm-455 Buicks, Olds, and Pontiacs are perfect examples of cars that can run quick with 3.50:1 or 3.73:1 gears. Also, nitrous allows the use of higher gears, not only because it increases torque dramatically, but because it causes engine rpm to increase at a rapid rate. Add nitrous and you may find yourself shifting earlier, and if you're in top gear at half-track, then the engine will be screaming at the finish line. Using higher gears helps the nitrous work better under a load and also helps keep the engine in its powerband all the way down the track.

Here's a reason low gears can be beneficial to dragstrip performance that you've probably never considered. Assume that a car is in a 1:1 trans ratio, has 26-inch-tall tires and 3.08:1 axle gears. When accelerating from 50 to 70 mph, engine speed increases by about 800 rpm. Put 4.10:1 gears in the same car, and engine speed increases by 1060 rpm-the difference is 40 rpm per mph with the 3.08s versus 53 rpm per mph for the 4.10s. The greater rate of rpm increase versus road speed provides greater acceleration. Since horsepower increases as engine rpm increases (up to the point when the torque curve drops off at a greater rate than engine rpm increase), the engine is able to overcome loads more easily with lower gears than with higher gears. This helps not only in acceleration but in maintaining road speed under a load such as when climbing a steep grade.

If you prefer top speed to dragstrip performance, then higher axle gears, like 2.76:1 or 3.08:1, may be called for. The higher gears will reduce engine rpm versus road speed. Another way to look at it is that the car will go faster at the engine's rpm limit than it would with lower gears. Low-speed acceleration will suffer, but that can be cured with one of today's manual five- or six-speed trannies that provide lower First and Second gears for acceleration and also feature overdrive for even more top speed-or engine-rpm reduction, depending on how you look at it.

Why Does Tire Height Affect Cruise RPM?
Sometimes you'll hear people talk about "effective gear ratio" to explain the drop in cruise rpm after installing taller tires or the increase in rpm with shorter tires. Here's their theory: If a car starts with 3.50:1 gears and 26-inch-tall tires, but the tires are then swapped to 30-inchers, then the effective gear ratio is 3.08:1. In other words, the cruise rpm with 3.50:1 gears and 30-inch tires is the same as it would be if the 26-inch tires were retained and 3.08:1 gears were installed.

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14 comments
Jaseac
Jaseac

Really wish you offered a "PRINT ENTIRE ARTICLE" option. Can print this thing without doing a print screen function

David Dony
David Dony

Can you imagine !! that 98 to 14 it's like going to 68 to 84 ????

James Salisbury
James Salisbury

3.90s and a 10 inch converter are lousy in Phoenix traffic.

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