How to determine rear axle ratio and choose the best differential gear
Today we’ll discuss how to select the best axle gear ratio based on the tire size that you’re running on your 4 wheel drive.
Before we get into what ratio you should run in your axles, let’s get to the basics and talk about what exactly the gear ratio is. There is a pin and ring gears in your differential. The pinion gear is attached to the drive shaft of your vehicle. Pinion gear engages the ring gear and your axles are attached to your ring gears through a carrier. Pinion is a drive gear and ring gear – is driven gear. Gear ratio is the number of teeth on the pinion gear compared to the number of teeth on the ring gear.
HOW TO DETERMINE YOUR GEAR RATIO?
1. Count the number of teeth on your pinion
2. Count the number of teeth on your ring gear
3. Divide the number of ring gear teeth to the number of the pinion ring teeth.
For example, you got 11 teeth on the pinion gear and 41 on the ring gear.
41 / 11 = 3.72727273 round it to two digits after the decimal point and you get 3.73
Another way to determine your gear ratio is to look at the tag on your differential cover. It should state the number of teeth on the pinion and ring gears, but often this information is hardly readable because of the rust or possible damage.
On most of the gear sets the ratio is stamped anywhere on the parts. In case you don’t see a stamp just use a counting method described above.
Why do we need to care about the gear ratio?
When your truck was designed at the factory, the engineers have taken to account a lot of different parameters such as engine type, transmission, rear axle type and wheel size. Their goal was to select the most efficient setup for the engine operation to provide good balance of power, speed and fuel economy. Installation of large wheels changes the entire equation and we can often see changes in the work of automatic transmission, which sometimes fails to shift upwards or go to overdrive when it’s needed. It also causes a lot of problems with performance and fuel efficiency, so gears in your differential play a very important role.
HOW TO CHOOSE THE BEST GEAR RATIO FOR YOUR NEW TIRES?
New tire size + Old ratio divide by old tire size = new ratio
Example:
New tire size – 33”
Old gear ratio – 3.07
Old tire size – 29”
(33 * 3.07) / 29 = 3.49
Even though the ideal gear ratio may not be available, we should pick the closest options which turn to be 3.55. Additionally, if you run an automatic transmission, you don’t have to worry about shifting.
One other thing that you have to know about ring and pinion kits is that they are manufactured as precision pairs, which means that they are not interchangeable with parts from other kits even with similar size. You can’t pick a ring from one set and match it with the pinion from a similar kit, otherwise, it will result in extensive wear and noise. If you buy a used set, you have to make sure that both components were designed to be used together.
THE MOST COMMON GEAR RATIOS:
- 3.07
- 3.55
- 4.10
- 4.56
- 4.88
Lower the number – higher the gears. 3.07 gears will give a perfect speed and fuel efficiency on a highway, but it will have a very poor torque while crawling or going of rough terrain. The situation is directly opposite with 4.88 gears that are excellent in the lower range but don’t give out a high speed on the concrete roads and decrease the gas mileage.
Very well written, thanks for posting this!