Effective Gear Ratio - how your current gear ratio will feel like after a tire size change

(old tire diameter / new tire diameter) * axle ratio = effective gear ratio

Equivalent Ratio Needed - approximately the gear ratio you'll need after a tire size change to equal the feel you have with your current gears and tires

(new tire diameter / old tire diameter) * original axle ratio = equivalent ratio needed

Engine RPM Prediction - estimate what your engine rpms will be at a given speed after you change tire sizes

(mph * gear ratio * 336) / tire diameter = engine rpm

Speed Prediction - estimate what speed your vehicle will be travelling at a specific rpm

(rpm * tire diameter) / (gear ratio x 336) = speed (mph)

Actual Speed from Indicated Speed - what your actual speed is compared to the speed shown on your speedometer after a tire size and/or gear change.

TIRE CHANGE ONLY: (new tire diameter / old tire diameter) * indicated mph = actual speed

TIRE AND GEAR CHANGE: (new tire diameter / old tire diameter) * (old axle ratio / new axle ratio) * indicated speed = actual speed

Determining gear ratio - count the teeth on the ring gear (little) and the pinion gear (big). Divide the pinion gear count by the ring gear count.

For example, if you have 39 teeth on the pinion gear and 11 teeth on the ring gear, your ratio is 3.54

Generally speaking, it's recommended you purchase a winch with a pulling capacity of 130% of your vehicle weight. However, there are other factors besides pure weight that affect winch pull.

To help determine additional line pull requirements consider the following:

Additional pull needed for ground conditions:

Since you may be pulling yourself or others up a slope, you need to factor that into the equation as well. Generally speaking, you can take the approximate degree of the slope, divide it by 60 and then multiply that by the weight of your vehicle (while loaded).

(slope (degrees) / 60) * vehicle weight (loaded) = additional winch pull needed

Add that to your fully loaded vehicle weight and add on any of the percentages above that apply to estimate how large a winch you'll need.

For example:

Let's say your Jeep weights 5000 pounds with you and your gear inside. You're stuck going up a gravel trail at approximately a 30 degree slope. This would be your calculation:

(30 / 60) * 5000 = 2500 additional pounds to account for slope.

2500 + 5000 = 7500 pounds total winch pull

7500 * 1.20 = 9,000 pounds when factoring in ground conditions

If you have a winch that pulls less than 9,000 pounds, you can often double or triple the line pull to make up for the difference. Keep in mind this slows the line speed down considerably.