There are many reasons why you would want or need to change gear ratios in your vehicle. I could probably write pages on all the reasons for a procedure like this, but I will try to summarize it for you. My focus when consulting with customers about regearing tends to center around engine rpms. When using gear calculators the number that you should be watching change is the engine rpms. If you look at a ratio like 3.73:1, this means that for every 3.73 rotations of the pinion gear you get one rotation of the ring gear. So, if you change your ratio numerically up or down, you are simply changing the number of times the pinion must turn to achieve one rotation of the ring gear. This translates directly to how many times the engine has to turn(rpms) to maintain any speed. If you have a truck, Jeep or car and you’ve noticed that you cannot stay in overdrive, the transmission is gear hunting while on the highway, transmission oil temperatures are high, a feeling of loss of power or engine rpms are too high, you are likely a good candidate for a gear change. If you simply want more performance out of your vehicle, faster acceleration, better towing capacity or off-road performance you are likely a good candidate for a gear change.

Horsepower and torque are not a replacement for a gear ratio change! Unless you are building a race car the amount of power your car or truck makes is not the solution for improper gearing. In fact, this may simply cause more damage and wear on everything behind the engine. 100 or 1000 hp is not going to change what your rpms will be at. So, if the rpms are too low at highway cruising speeds it will kick out of overdrive and pick up engine rpms to get back to the speed you want. With more power you may reduce the number of times your transmission has to change, but all that means is the transmission is taking more load and building more heat, which is the worst enemy of a transmission. With the proper gear ratio, the vehicle will work in harmony, giving you the best performance and economy.

While both devices are traction aids and are offered in several different designs, the way they function and provide traction are very different.

A posi-traction(limited slip) differential divides power to both tires while under load. Depending on how much traction each tire is achieving will determine whether or not the posi unit will be able to overcome that force. In most street and light trail applications, a posi-traction unit can send power to both tires. However, it’s not always an even 50/50 split. It’s called “limited slip” for a reason. If you could see the tires spinning in slow motion, one tire will likely be moving faster or slower than the other. There are several designs for these units. The most common is the clutch style which uses several clutch plates stacked behind the side gears. The spider gears are tapered, so under torque load they are pushed back which applies pressure to the clutch plates, and much like a clutch in a manual transmission, the friction material grips to itself and attempts to stop the spider gear from rotating. This results in power being sent to the other tire with more traction as opposed to it going all to the tire with no traction. The next most popular posi unit is the helical. It has no clutches and instead uses spur gear and worm gears instead of clutches. These are considered torque biasing, which means it will feed power to the tire with the most amount of traction. These types of posi units are ideal because they are stronger, grip harder, have no wearable components, and don’t require any special additives. For this reason, they are a bit more expensive, but tend to be worth the added cost. They are also very effective in front differential applications due to the fact that under light torque loads they operate smoothly (almost like an open carrier), with no additional wear on the parts. There are many other types of posi units, clutch and helical. Depending on your application either can be a good choice.

Frequently asked questions