Retrofit classic car rev counter

Today, it's hard to imagine a vehicle without a rev counter, but in the past it was not a very widespread optional extra. Unless you drove a sports car, which had them as standard equipment early on. Retrofitting classic cars with a rev counter is, however, easily possible, even for non-professionals. Here you can find out about the advantages of retrofitting, the difference between electrical and mechanical devices and how to proceed.

Author: Max Berger
Position: Sale
Updated: 20.10.2023

Retrofitting a rev counter: Basics

First things first: When it comes to retrofitting a tachometer, it must be said that this only applies to electric tachometers. You either have a mechanical tachometer or you don't. In addition, it is already fitted to most English classics anyway. If you want to retrofit a rev counter on a classic car, you will have a continental classic in the garage waiting for an electric rev counter.

We also have some good news to share with you: Retrofitting is a feasible undertaking for non-professionals! The only thing you have to take care of is to install a rev counter that matches the vehicle and its number of cylinders. And the device should also match the appearance of your vehicle. Despite the low prices, you should not install colourful Chinese models out of respect for the vehicle's cultural heritage.

What is a rev counter?

A rev counter, as the name suggests, measures the number of revolutions. The RPM, in turn, indicates how often the crankshaft rotates around its own axis and does so exactly within one minute. It is given in u/min or rpm. For classification:

Aircraft engines rotate at 2,500 rpm
older English sports cars turn up to 5,000 rpm
Italian sports cars partly 8000 rpm
Motorbikes also sometimes rev up to 15000 rpm

Why do you need a rev counter?

Internal combustion engines have two important key data. The first is the maximum power. This is always at a design-related speed. It does not make sense to rev beyond this speed, as it does not produce more power. The second key figure is the maximum torque, which is also present at a fixed speed for design reasons.

The knowledge of these two revs and the relation to the current revs is the most important and only relevant information in motorsport. The largest, centrally located and most easily read instrument is therefore the rev counter. Everything else is secondary or irrelevant. The rev counter has another vital feature: the red line.

This red line is a limit that must not be exceeded. The engine designer has set this red line in the knowledge of the valve springs installed, the design of the camshaft and the piston speeds applied. Ignoring this red line can quickly lead to serious engine damage. The rev counter, also known as the tachometer, is found on most cars in the dashboard near the speedometer.

For the non-sporty driver, the tachometer is basically an instrument that helps to ensure proper engine operation and assists the driver in shifting gears "correctly", conserving the engine and optimising fuel consumption economically.

Use of tachometers

In short, a tachometer is used to indicate whether the current operation of the engine is safe or damaging. To do this, you use the scale on the rev counter, which is divided into several zones. Usually colour-coded in green, yellow and red. Because each engine has specific maximum speed limits, these zones are of course always adapted to the parameters of the unit.

Retrofitting a rev counter in the speedometer or as an additional instrument?

In most cases, this works both ways and the decision is either one of visual wear or one dictated by the available space. For most classic cars simply have no space provided for the instrument, as tachometers were hardly a standard feature in continental Europe's vehicles 40 years ago.

Sometimes you can install the rev counter in place of an existing clock in the cockpit, which you simply remove instead. In most cases, however, retrofitting can only be done in the form of an additional instrument near the speedometer or the steering wheel. There are universal housings that can be installed either on or under the dashboard. Take a look at the Limora Warehouse.

Mechanical and electrical versions: Differences

A distinction is made between mechanical and electrical tachometers. As already noted above, retrofitting is only possible on electric models. Both variants differ fundamentally in their mode of operation, i.e. how the speed is measured.

In the mechanically driven rev counter, a flexible shaft turns a permanent magnet. The shaft is usually powered by the engine control unit. The rotary motion generates eddy currents which the indicator uses to display the speed.
Electric tachometers, on the other hand, work as follows: With this variant, a pulse is taken from the ignition coil. Depending on the number of cylinders, this results in the number of revolutions. This is done electronically, inexpensively and inconspicuously.

From theory to practice: What to do when retrofitting a rev counter?

Once a suitable console has been found for mounting in the interior, select a tachometer to match. Most consoles offer space for 2" devices, the measurement is international standard. Only 2 cables need to be attached to the tachometer, which has a diameter of 51 or 52 mm:

A cable is connected to terminal 15, ignition plus. This can be found professionally on the fuse box, or tinkered with on the radio. This is how the new rev counter gets its operating voltage. If the housing is made of plastic and the rev counter does not have a ground, an additional ground cable must be laid.
The second cable, which gives the unit the impulse, is connected to terminal 1 on the vehicle side, i.e. to the ignition coil.
With some units, the lighting can be connected to the vehicle lighting. If this option is used, another cable is added, usually at terminal 58.

Finally, set the number of cylinders and the tachometer retrofit is complete. But beware: this does not work with diesel engines, of course, for which there are separate, but technically much more complex solutions.

Retrofit tachometer for classic car

If you are looking for a tachometer that fits perfectly into your classic car, both visually and technically, then Limora is the right place for you! We have a large selection of Smiths tachometers and also the matching accessories for tachometer retrofitting in classic vehicles.

If you have any questions, you can also contact our experts without any worries. So get started, get to know your engine better and save fuel to boot. All thanks to the right rev counter.