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How to Determine GMC Gear Ratio

The gear ratio of a vehicle determines whether the vehicle will have more low end power or high end speed. A vehicle that has a lower gear in the rear differential usually gets less fuel mileage, whereas a vehicle with higher gears tend to get better fuel mileage because the engine turns less revolutions per minute. The lower the gear ratio number the higher the gear ratio. Most manufacturers gear their vehicles for maximum performance and mileage. To determine the gear ratio of a GMC or any vehicle the task is accomplished the same way.

Instructions

Raise the Vehcile

1

Park the vehicle on a level surface. Insert two wheel blocks, one on the front side of each front wheel.

2

Place a floor jack in the center of the rear differential and raise the rear wheels off the ground.

3

Set two more wheel blocks, one on the back side of each front wheel, to ensure the vehicle cannot move.

4

Spot two jack stands, one near each wheel, under the rear axle tubes.

Determining the Gear Ratio

5

Wrap a thin, flexible tape measure around the driveshaft tube near the rear yoke. Accurately measure the circumference of the shaft. Record the measurement.

6

Clamp a piece of coat hanger wire to each of the remaining two jack stands to use as a pointer. Use one jack stand as a pointer placed next to the rear tire. Make a corresponding mark on the tire sidewall that aligns exactly with the wire pointer on the jack stand. The pointer is needed to accurately record exactly one wheel rotation.

7

Clamp the end of the 16-gauge yellow electrical wire tight to the driveshaft with a hose clamp around the 6 o clock or 9 oclock position on the shaft. Bend and pull the yellow wire down perpendicular to driveshaft so the wire will be drawn up when the shaft rotates.

8

Mark the yellow wire, with a permanent marker, where it comes out from under the hose clamp -- this will be the starting point. Spot the other jack stand next to the driveshaft with the wire pointer aimed directly at the starting point mark on the wire.

9

Turn the key on, just enough to unlock the wheel and place the shifting lever in neutral so the driveshaft can be rotated.

10

Rotate the driveshaft in the direction that winds the yellow wire up perpendicularly onto the shaft like a winch cable on a winch. Keep the wire tight and the wraps tight to each other.

Stop rotating the driveshaft when the wheel turns exactly one turn. Make a mark on the last wrap of the yellow wire that aligns with the pointer on the jack stand near the driveshaft -- this will be the end mark.

11

Unscrew the hose clamp from the driveshaft. Remove the yellow wire and measure the distance accurately between starting mark and the end mark.

12

Divide the length of the yellow wire in inches by the length of the driveshaft circumference in inches; that figure is the gear ratio.

For example: The wire measured 42.375 inches. The circumference of the driveshaft measured 10.625 inches.

Divide 42.375 by 10.625-inches and it equals 3.988. Rounded off it makes a 3.99 gear ratio, which means that the driveshaft turns 3.99 times for each one turn of the wheel.

13

Remove the tools from under the vehicle and the jack stands from under the rear axle tubes. Lower the vehicle back onto the ground.

Posted by Unknown at 11:30 AM

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