I'm looking for a tachometer up to 4 inches diameter and 6000rpm but with 25rpm resolution.

I need this for a regularity rally and the rules allow a gauge (could not be digital) up to 4" and min 6000rpm.

What is not stablished is the resolution. Does anybody know of some tachometer with this specs¿

thank you very much!

25rpm is very very small, 240 lines around the dial.
Do you mean 250 rpm resolution?

The resolution of a good quality, fully analog tachometer is limited by the thickness of the needle and your own ability to distinguish very small movements. It's only when you use a stepper motor or other digitally controled movement or have a digital display that the resolution is limited by design.

The ultimate in analog tachometers is probably the "Chronometric Tachometer" that actually counts physical engine rotations within a fixed time period rather than looking at the period between ignition events, but they are very expensive and require a cable drive to the engine.

Surely you won't be able to physically control the engine to with 25 rpm for any significant length of time?

[Edited on 4/4/11 by MikeRJ]

Actually tachs with this resolution are made in Argentina, but build quality is terrible:



Some people also use magnifying lenses over the tachometer.

I've been involved with this type of competition for some time and it's not unsual to complete a 3 day competition and more than 25 hours driving in open roads with an error of less than 30 seconds total, so I would say, they can really hold engine revolutions for very long periods

Thanks.

Aren't you allowed to use a watch?

Yes I use 3 watches with 3 rows displays.

But you don't know the distance between references (a reference is where you need to record your time).

You know you have to drive at say 47 miles during a prime, so you get 4-6 known references with distance and time. You can adjust you speed at this time to do 47 miles using the stopwath (we are using 1/100th of a second here). This is known as the search (search for you speed) zone.

Then you enter the controlled zone where you get the reference (a tree, a traffic signal, etc) but you don't know the distance. Only by doing 47 miles (in this example) you can record the time in a globally synchronized clock that is used by the organizers to check how close you are...

The 3 watches are: 1 for searching speed, 1 for counting time where you are not doing the actual speed (for example when overtaking) and the other for backup of the first triggered at the same time.

Some would say a GPS would work, but by today competitors standards it's of no use..

Thanks again!

It seems to me a much more useful design would be to allow you to enter your target RPM, and the scale would be centred around this value giving you maximum possible resolution i.e. you enter 3000RPM and the middle of the gauge becomes 3000RPM, to the left (anti-clockwise) would be maybe -500RPM and clockwise would be +500RPM. This would be pretty easy to do with a digitally controlled gauge with an analog display.