RPM

[abri]

I recently installed a Smiths RVI 1000/00 tacho in a car I believe has its standard final drive of 3.44 and 10" wheels.

I get:

55km/h at 3000rpm
75km/h at 4000rpm
97km/h at 5000rpm
These km/h figures are off a GPS, so they are accurate.

According to the bible I should be getting
51.4km/h at 2000rpm
102.8km/h at 4000rpm
etc.

Clearly something is wrong. Even if I use the 4.35 final drive figures I only get 80.9km/h at 4000rpm.

Is there anything one can adjust to lower the RPM reading of the tacho?
Is there a way to confirm what final drive you have on your car without removing the motor from the car?

[Frogeye61]

You will get some deviation from different tire sizes, but...
You seem to think the tachometer is reading wrong. This can be checked at a local auto shop on one of their digital meters.
The figures you give indicate diff ratio of 4.7 which could be 4.35 with smaller tires. (3.44*102.8/75)

On a vehicle with a visible propeller shaft it is easy to measure the diff ratio. Not so easy on the Mini.
While rolling the car in 4th gear, Count engine revolutions / transfer ratio, count wheel revolutions.
Good luck.

[Number Six]

Hi,

Check http://www.guess-works.com/Tech/ratio.htm

Regards

André

[mini129]

For a 3.44 diff and standard tyres you should get 1280 revolutions per mile or 800 per km from the speedo cable.For 3.776 its 1408. Count the cable revolutions over 10 or 20 metres and multiply out to see what ratio you have in the box.
You can adjust the tacho with a blade screwdriver. It should be 100hz= 3000rpm for 4cyl

[abri]

Thanks for the advice guys. I've checked the speedo cable rotations over 10m. I get 8 rotations, which means the car's original 3.44 final drive still with it, further supported by the 1280 speedometer being pretty close to spot on compared to a GPS.

I guess this means my tachometer is off. I assume the adjustment happens under the little rubber grommet filled hole on the back?
I've opened a spare tacho and checked in the back, where it does appear it can be adjusted with a screw driver. However, there are no numbers or anything to indicate what the current setting is and which way it should be turned.

How do I measure the 100hz?

[Frogeye61]

Go to the local technical college and get one of their students with a frequency generator to test and adjust your tach.