MRW MANUFACTURERS OF "CUSTOM BUILT" MOTORSPORT WHEELS SINCE 1955

TECHNICAL CHARTS

  Gear Ratios

Gear ratio is a number, usually carried to 1 or 2 decimal points, representing how many turns it takes of the input shaft to make one revolution of the output shaft. this applies to transmissions, power take offs, speed or power gear boxes, and rear axles.  If the input gear is turning slower than the output gear then the system is said to have a speed ratio. If the input gear is turning faster than the output gear, the system is said to have power ratio.

In most  simple gear arrangement, the gear ratio  can found by dividing the tooth count of one (smaller) gear to the tooth count of other (larger)  gear.  

One special case of gear ratios are the engine speed of the car to the rotation of the drive wheels. In top gear, one turn of the engine crankshaft results in one turn of the driven shaft minus the reduction of the pinion to the ring gear ratio "final drive"  to make one revolution of the tire. (exception) == (automatic transmissions torque converter slippage changes engine to tire ratio) 

Lower gears require more turns of the engine to provide a single turn of the drive wheels, producing more torque at the drive wheel.

  If the drive gear is smaller than the driven gear then it is said to have power ratio and will develop more torque to the driven shaft.  On the other hand. If the drive gear is larger than the driven  gear then it has speed ratio with less torque to the driven shaft.

An example of a automobile transmission gear ratio 

A final drive ratio of approximately 2.8 to 1 is a commonly used gear ratio, in cars with an automatic transmission. This means that the drive pinion (small gear) must rotate 2.8 times to make the ring gear (large gear) rotate one time. On cars with manual transmissions more torque power ratio is needed, generally a ratio of approximately 3.5 to 1 is used. Small engine cars and trucks use a final drive ratio of up to 4.5 to 1 and higher  to provide even more torque to enable them to pull or move heavy loads. Also  shifting to lower gears in the transmission  requires more turns of the engine to provide a single turn of the drive wheels, producing more torque at the drive wheels.

"EXAMPLE"    A  transmission  and a 3 to 1  final drive 

  
First gear                   3 to 1 + 3 to 1  = 9 to 1 = max. torque

  Second gear        2.5 to 1 + 3 to 1  = 7.5 to 1

  Third gear            1.5 to 1 + 3 to 1  = 4.5 to 1

  Fourth gear             1 to 1 + 3 to 1  =   3 to 1

  Overdrive        0.75 to1  + 3 to 1    =  2.25 to 1 = max. speed

  Advice to help choose the gear ratio for your race car. One would be wise to talk to one who has raced the track preferably a winner. There are many variables that determine the gear ratio best for your car -  Weight of the car, size of tires, length of track, condition of track, weather, track rules, max h.p. to rpm curve, and many other factors you will need to consider. Even with all variables considered, you will still have to run the track many laps and change several times to get it right. Good Luck 

   Advice for choosing the gear ratio for Off road Rockcrawling, mud, bog, woods, sand, tundra, etc,   One would be wise to talk to one who has run the coarse preferably a winner also. Good Luck

MOTOR TO REAR END GEAR RATIO CHART

This chart is based on 65 MPH with a gear ratio of 1to1, on a standard transmission in high gear without overdrive. If you have an automatic transmission your RPM will be higher due to slippage of the torque converter. With an overdrive transmission your RPM will be less in overdrive than this chart indicates.

For approximate gear ratio multiply .12 by your tire diameter.
(.12 X 30" = 3.60) -  finds approximate gear ratio.

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MRW RACING WHEELS BEADLocK ROCKCRAWLER  AND OFF ROAD WHEELS

MANUFACTURERS OF "CUSTOM BUILT" MOTORSPORTS WHEELS SINCE 1955

MARSH RACING WHEELS  16951 LOGAN CAVE ROAD SILOAM SPRINGS, ARKANSAS 72761

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