How to Determine Gear Ratio by Spinning One Wheel
A vehicle's gear ratio is the speed of rotation of the propeller shaft (or drive shaft) in relation to the rear wheels. In most cases, the engine and transmission spin the propeller shaft much faster than needed, so the vehicle's differential assembly slows down that motion to a more manageable speed by way of ring and pinion gears. The most accurate way to determine a vehicle's gear ratio is to count the teeth on these gears, however, the differential would have to be disassembled to accomplish this. There is a must easier and quicker way to determine your gear ratio by spinning one rear wheel.
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Instructions
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Park the vehicle on a firm, level surface and place wheel chocks under the front wheels of the vehicle to prevent any unwanted movement. Raise the rear of the vehicle with a hydraulic jack and support the axle housing on each side of the vehicle with sturdy jack stands. Lower the hydraulic jack.
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Crawl under the vehicle and mark a horizontal line about two inches long on the propeller shaft with a piece of chalk.
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Mark the bottom of one of the rear tires at the 6 o'clock position with a piece of chalk.
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Use the chalk mark on the rear tire as a reference and begin rotating the tire by hand from a position that allows you to see the propeller shaft.
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Turn the rear tire one complete revolution while simultaneously counting the number of revolutions of the propeller shaft indicated by the horizontal chalk mark. Be as accurate as possible.
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Determine your gear ratio, which is the number of propeller shaft revolutions that were made in relation to one tire revolution. For instance, if your propeller shaft rotated three and one half times in relation to one tire revolution, your gear ratio is 3.50.
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Use the hydraulic jack to raise the vehicle high enough to remove the jack stands from under the axle housing. Lower the hydraulic jack and remove it from under the vehicle. Remove the wheel chocks.
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Tips & Warnings
Further pinpoint your actual gear ratio by checking your vehicle owner's manual to determine what gear ratios were available for your vehicle. Compare these ratios with the results of the test you performed above. For instance, if your vehicle was available with gear ratios of 3.08, 3.55, and 4.11 and the result of your test was three and one half propeller shaft rotations, chances are your gear ratio is 3.55 since that is the closest ratio to your result.
Make sure the jack stands you are using can safely support the weight of your vehicle. For smaller cars, one-ton jacks would be sufficient, but if you are supporting a ¾ ton pickup, use two- or three-ton jacks.
References
- Photo Credit wire wheel image by Edsweb from Fotolia.com
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Comments
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Hi - The title to this otherwise fine piece is rather misleading. The technique described will only find the *rear differential* ratio, not the 'gear'-box ratio. Suggest you add 'Differential' before the word 'Gear' or replace the latter with the former to make it clear. Regards, MW.
