Classic Instruments 3-3/8- and 4-5/8-inch electric speedometers read an electrical signal ranging from 11,000 to 110,000 pulses per mile. Speedometers are calibrated by resetting the "DIP" switches after a road test. The odometer counter then re-synchronizes to the new, corrected pointer reading.

Signals for electronic speedometers can be generated a variety of ways:

SN16 pulse signal generator: used on GM or Chrysler transmissions that have a mechanical speedometer cable drive. The pulse signal generator replaces the speedometer cable and produces a "square wave", which alternates between 0 and +12 V. The SN16 is a powered signal generator, which means it requires +12 V to operate, unlike some two-wire pulse signal generators that create their own voltage.

SN17 Ford pulse signal generator adapter: used to convert the connection from Ford and similar transmissions that have a mechanical speedometer drive cable to work with the SN16.

SN74 speedometer signal interface: used to convert speed signals from electronic transmissions or vehicle computers to one that is compatible with the Classic Instruments speedometer. Calibration couldn't be easier than simply a matter of pushing a button. Some Classic Instruments speedometers have the SN74 functionality built into them with no signal interface needed.

SN81 GPS speedometer signal: Called the Sky Drive, it uses GPS satellite signals to operate an electronic speedometer. It updates 10 times per second and eliminates the need for ECU convertors or pulse signal generators screwed onto the transmission. SN78SM magnetic proximity speed sensor: This sensor has a magnetic head, which senses targets attached to the drive shaft or brake rotor. The SN78SM will provide a +12V pulse upon detection of each target.

Fuel Gauge
The most common Classic Instruments fuel level sender has a resistance range of 240 ohms empty and 33 ohms on full. Other senders available operate in resistance ranges of:

~0 to ~30 ohms (typically used with original GM fuel gauges before 1964)

~0 to ~90 ohms (typically used with original GM fuel gauges after 166)

~78 to ~10 ohms (typically used with original Ford fuel gauges before 1986)

Classic Instruments offers fuel gauges that work with these senders and more (check with them for gauge/sender compatibility).

We should add that a good ground at the sender is necessary for the fuel level gauge to operate properly. Don't rely on the tank to provide the ground, as most are electrically isolated—run a dedicated ground wire from the sender to the chassis. A bad or missing ground will make gauges with 240-33 and 75-10 operating ranges to peg below empty—0-30, 0-90, and 16-158 fuel gauges will peg above full.

Oil Pressure Gauge
Classic Instruments oil pressure gauges require the proper sender for the range of the instrument (0 to 80, or 0 to 100 psi). The lower the resistance, the higher the reading on the gauge. Because of this, the sender must be properly grounded to provide the proper signal or the gauge will read low or peg below 0 psi. Pipe sealer should be used sparingly on the threads of the sender and Teflon tape should not be used.

Temperature Gauge
Temperature senders should be mounted in the intake manifold of the engine when possible. Installing the sender in the head or in close proximity to other heat sources (i.e. exhaust headers) will cause the temperature gauge to read high. Additionally, bushings should not be used to make a temperature sender fit as they can cause the sender probe to not be fully submerged in the coolant (this could cause the sender to measure coolant vapor temperature, which is usually higher than the actual coolant temperature). All Classic Instruments temperature gauges require the correct sender—they are available to fit most stock or aftermarket intake manifolds.

Volt Gauge
The volt gauge does not require a signal like the temperature, oil pressure, and fuel level gauges do. The volt gauge is simply connected to a good ground and switched battery power.

Although Matt did not opt for a tachometer, when one is used it can get a signal from a computer, ignition module, or coil. Some signals require an adapter to work correctly.

The new instruments made it necessary to install the proper oil pressure and temperature senders, however the original fuel tank sender was retained, as it was compatible with the replacement gas gauge. An electrical source for the voltmeter was needed, an adapter cable for the speedometer's pulse generator was necessary and then it was simply a matter of reinstalling the dash panel.

With the gauges in place and the new senders installed Matt's 1956 has a real dash of class with a vintage look he was after along with the reliability and accuracy of state-of-the-art gauges. And there are no idiot lights to tell him too little too late.

Classic Instruments
1299 M-75 South
P.O. Box 411
Boyne City
MI  49712