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12-18 Edition 47
On-Highway Trucks
Selecting Gear Ratios for Transmission and Axles
SELECTING GEAR RATIOS FOR
TRANSMISSION AND AXLES
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.
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 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.
MPH (cruise) =
RPM (at cruise) × 60
AR × TR × M × ATR × DBR
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).
Suggested Cruise Speeds
Engine Family
Application
Engine Speed for
Fuel Economy
Engine Speed for
Performance
Vehicle Cruise Speed
CT13
Vocational
1375-1500
1500-1650
55
CT13
Extended Highway
1325-1375
1375-1450
65