Caterpillar Performance Handbook, January 2017, SEBD0351-47

Selecting Gear Ratios for Transmission and Axles

On-Highway Trucks

SELECTING GEAR RATIOS FOR TRANSMISSION AND AXLES

RPM = Engine RPM at desired cruise. See table below or Torque Curves in Appendix C. TR = Transmission ratio (Top gear). See Appendix D. MPH = Desired cruise speed (not maximum speed). M = Tire revolutions per mile. See Appendix I. ATR = Auxiliary Transmission ratio. See Appendix D. DBR = Drop Box Ratio. See Appendix D (omit if 1:1). 60 = Constant.

Select the Desired Axle Ratio (AR)

AR = RPM (at cruise) × 60 TR × MPH × M × ATR × DBR NOTE: This equation provides the Theoretical axle ratio. Select the closest axle ratio found in Appendix E.

Suggested Cruise Speeds

Engine Speed for Fuel Economy

Engine Speed for Performance

Engine Family

Application Vocational

Vehicle Cruise Speed

CT13 CT13

1375-1500 1325-1375

1500-1650 1375-1450

55 65

Extended Highway

Select the Desired Transmission Ratio (TR) Top Gear/Overdrive TR = RPM (at cruise) × 60 AR × MPH × M × ATR × DBR NOTE: For AR, use the Axle ratio selected fromAppen- dix E. NOTE: This equation provides the Theoretical Trans- mission ratio. Select the closest actual gear ratio from Appendix D.

Verify Cruise and Maximum Speed Use the actual Transmission and Axle ratios you selected from Appendix D and E to verify the desired cruise and Maximum speed. Do not use the theoretical values. If you’re within 5 mph of your desired cruise speed that is a good gearing combination. Otherwise, modify your choices for the Axle or Transmission Ratios to get closer. It may take a few attempts to meet your requirements.

RPM (at cruise) × 60 AR × TR × M × ATR × DBR

MPH (cruise) =

MPH (Max) = 2100 (max RPM) × 60 AR × TR × M × ATR × DBR NOTE: Use the actual AR and TR selected fromAppen- dix D and E for MPH (cruise) and MPH (Max).

12-18 Edition 47