There you are, you've just plonked that dirty big injected V8 engine into the nose of your pride an' joy, bolted up a high ratio diff and maybe a set of different diameter wheels and tyres. Everything's gonna be sweet, right? Well, unfortunately, that might not be true. There's one area that's often overlooked when performing such mods - the resulting inaccuracies of the speedometer and tachometer.
And even if you're not quite as adventurous with vehicle modifications, you may one day encounter problems with a factory speedo that reads extra fast, or a tacho that tells pork pies. To solve either case, we went along to Adelaide's Beale Instruments to discover how to overcome these problems with a minimum of fuss...
First of all, there are two different types of speedometers - mechanical and electronic. Mechanical speedo dials function which use a magnetically rotated main shaft (that's hooked up to the needle) are the most prone to requiring recalibration. Electronic speedos work by receiving a series of electronic pulses that the IC board converts into a voltage that moves the needle. These are generally fitted to most modern vehicles (usually post-1980s).
When a mechanical speedometer is suspected of being inaccurate, Beale Instruments check it against a Smiths Speedometer Calibration machine.
If the speedo does come up with a false reading, it is either partially magnetised or de-magnetised accordingly. For example, to trim downwards the speedo's indicated speed, the magnets at the back of the gauge are moved up against a de-magnetising block built into the calibration machine. This reduces the amount of magnetism on the gauge's main shaft, and the needle therefore doesn't travel as far.
As you'd expect, an electronic speedometer needs to be tackled in a completely different way. If it's the older type that doesn't have a trim-pot built in, it's necessary to solder in a replacement capacitor of a different value (which can only be determined by trial and error). However, if you're talking about a speedo from the last 15 or-so years, it'll most likely have an internal trimming potentiometer that can simply be screwdriver adjusted to give the right readings. It's that easy.
Note though that both of these types of electronic speedos very seldom need adjustment - if there's an accuracy problem, it may be caused by something else in the system.
Recalibration to Suit Gearing Changes
If the speedo unit itself is functioning accurately but you've altered the car's overall gearing (diff ratio or wheel diameter), you need a ratio box. The first step is to drive the car with a pulse counter plugged in to measure the number of speedo shaft revolutions over a measured distance (say, one kilometre). This number is then cross-referenced on a special chart and the right gear for the job is chosen from the list. It's then just a matter of putting the right gear set into the small ratio box housing and installing it to the cable between the gearbox and the speedometer input. These mechanical ratio boxes generally cost around AUD$110 retail.
If you've got an electronic speedo and you want to fix the same sort of problem, the process is the same. Except, in this case, the ratio box has an electronic output rather than a mechanical one. These transducers are usually fitted directly to the gearbox and give out the correct number of speedo pulses per shaft revolution, which gets passed on directly to the speedometer. These cost around AUD$75 each.
One interesting custom job Beale Instruments had on-hand when we visited was a VDO electronic speedometer integrated inside a VB Commodore binnacle - a change that was required to suit an injected V6 engine conversion. As you can see, the VDO dial fits into the cluster very neatly and, as a bonus, gives a LCD odometer display.
But this custom install did take a considerable amount of time. Firstly, the main board was cut to fit the new speedo and it was hard mounted to the facia. Running wires were then soldered in to connect parts of the existing wiring and solder tracks to the new speedometer. In total, this took around 6 hours, which meant the customer was charged $200 in labour.
Tachos (not Tacos!)
Tachometers all work on the same principle - they take a pulse input directly from the coil, or indirectly from an ECU.
Most late model Japanese cars use a sine wave signal, while VDO and a couple of other OE makers use a square wave signal. One important note is that most OE tachos after about 1990 have a rotary switch inside to make the tachometer gauge universal over a specific manufacturer's model line-up (to cut their production costs). This makes things easier if you're planning to install the most hi-po engine that came out in your car - just change the switch position to match the new engine. Older style tachos were designed to work solely with an engine of a preset number of cylinders and with a specific ignition system.
One of the most common upgrades in the auto aftermarket industry is to install a stand-alone tachometer such as VDO or Autometer. This is easy because these manufacturers' current line of tachos have an in-built switch that enables inputs ranging from 2-12 cylinders - some in either 2 or 4 stroke configurations.
However, if you're not keen on having a standout tachometer mounted inside the cabin, there is one option. An example of this is a job Beale Instruments recently performed, where a customer's original MG tacho dial had to be modified to suit an electronic ignition system. The tachometer assembly from a current Mitsubishi Mirage was used, since it shared a 300 degree sweep and had identically spaced numbers. The Mirage circuit board and meter was installed behind the original dial and was connected to the stock MG needle - keeping the conversion 100% original in appearance. It cost the customer about $100 in labour.
One common problem for both speedos and tachos is warped needles and pointers. Australia's harsh climate is known to take its toll on the needles of older European cars especially - just take a look at this needle pulled from a Peugeot (the one on the right).
In this example, a new set of donor pointers was sourced from a VB/VC model Holden Commodore. These slid straight on in place of the warped originals - the only other task was to re-calibrate the speedometer. This was necessary since the new pointer had a slightly heavier mass (which would cause low readings at the bottom half of the dial - and higher in the top half!). This job ended up costing the owner a measly $100 in labour. Not bad, considering the terminal damage you could do to the gauge trying to straighten the needles at home!
If you want to get tricky and paint your gauge needles a different colour, this can be done successfully so long as you are prepared to take it to an instrument specialist to have it recalibrated afterward. This is because the needle needs to be re-installed at EXACTLY the right angle - which is very difficult, unless you have the right equipment. Oh, and the extra weight of the paint shouldn't cause any problems in regard to needle accuracy - just don't slap on a heap of coats!
White-faced luminescent dials are also a bit of a hot item at the moment. These come in ready-to-go kits (mainly to suit late model Japanese cars) are able to be installed at home by most enthusiasts. These dials simply slide over the gauge needle and cover up the standard black faces - easy.
Another trick you might have already picked up on is installing aftermarket or different OE parts into your existing cluster. You can often fit different OE needles with your existing dials to add a subtle individual touch. Remember though, re-calibration will be required to maintain speedo accuracy. At a greater cost, you can opt for a full aftermarket set of gauges mounted behind the original fascia. This integrates all the gauges in the factory viewing location and there'll be nothing sticking out where there shouldn't be. You can also chose your own selection of gauges to monitor whatever you want.
Virtually anything can be done - it just depends what you're chasing and how much you want to spend.
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