MOTOR TRUCK SERVICE MANUAL
The odometer records up to 99, 999.9 miles and
automatically returns to zero.
There are occasions when, due to change in axle
ratio or tire sizes, it is necessary to make compensating
corrections in the speedometer drive gear. For method
of speedometer adapter calculations, see page 2 of this
A tachometer is installed on a vehicle to record
engine RPM speeds and enable the driver to keep
engine within efficient operating range.
The magnetic type tachometer is driven by a flexible
shaft connected to the vehicle distributor shaft by means
of a drive joint or adapter and records the RPM from the
distributor shaft on the face dial through a magnetic
coupling within the unit.
There are two additional types of tachometers
available, namely, centrifugal, with a maximum RPM
hand, and electrical.
The centrifugal tachometer works on the weight-type
governor principle and is driven from the distributor shaft
in the same manner as the magnetic type. The
centrifugal type also has a maximum RPM hand on the
tachometer head to record maximum speed of engine.
For diesel engines, the electric tachometer is a two-
unit combination consisting of a tachometer head and a
The speedometer unit is only serviced complete and
sender. The head is mounted on the instrument panel
includes the magnet, speedcup, cross gears and face
and the sender unit is mounted on the engine using the
tachometer drive take off.
A speedometer is used on a vehicle to indicate
SPEEDOMETER AND TACHOMETER CABLES
speed in miles per hour as shown by the pointer on face
dial and to record distance traveled by means of an
Two different types of speedometer and tachometer
odometer. The speedometer is driven through a flexible
drive cables have been used on IH vehicles. They are
shaft connected to a set of gears in the vehicle
the flexible shaft (wrapped type cable) and the semi-rigid
transmission. These gears are designed for the
types (covering of polypropylene or polyethylene on
particular vehicle model and take into consideration the
tire size and rear axle ratio. The flexible shaft, which
The flexible shaft cable consists of two major
connects the transmission driven gear to the
assemblies: a flexible outer casing
speedometer, consists of an outer casing and inner core.