I have calibrated the speedo, I know the diff ratio, how do I calculate how many revs the car should be doing in say 2nd gear at 30mph?

I need to calibrate the rev counter, any ideas?

Cheers Ben

What make / type tacho have you got ? may help to know . older gauges were usually for 4 /6/8 cylinders and didn't need calibrating , more modern gauges may have switchs inside which are set for number of cylinders and type of ignition system

I have got the telemetrix one.

It has been set for 4 cyl, etc with the dip switches, but has a variable resistor to set the revs accurately.

Unless someone has a better suggestion try to borrow a garage type rev counter to give you an accurate figure to go by and set resistor to match figures at several points through rev range

Just guessing really but could you do it using a timing light?

Cheers,
James

I can't count that fast James

If your engine timing light does not have a rev counter attach and run it to a car that does, point at your crank and when the speeds are the same your pulley will appear to stop spinning then match rev counters... unless that is its going at a multiple of the crank speed !

I got an excell sheet from this site that was originally posted by Mark allanson. You put in all your diff and gearbox ratios etc and wheel /tyre sizes etc and it gives you all the varios speeds in each particular gear at 1000 rev intervals. You could do a search for it or if you can't find it I could send it as an email attachment to you.

Engine speed = road speed * gear ratio * diff ratio / wheel radius

Engine speed must be in radians per second (=rpm * 2 * pi / 60 = rpm*0.10472)

Road speed must be in metres per second (=mph * 0.44704)

Wheel radius should be in metres.

Does that help or have I got the wrong end of the stick?

Pete

EDIT: In simplified form:

rpm = 4.2689 * mph * gear ratio * diff ratio / wheel radius

A good way to find the wheel rolling radius is to physically roll the car with the correct tyre pressures and measure how far the wheel travels over one revolution, then find the radius by dividing the distance by 2*pi

[Edited on 15/7/05 by scoobyis2cool]

Could i suggest doing it over 10 or more wheel revolutions with the car loaded as normal (ie you in it) and dividing the answer. It will be a lot more accurate.

try the quaife gear calculator, there's a download on the quaife website for it somewhere.

Ned.

ps http://www.quaife.co.uk/products/gearboxes.htm

[Edited on 16/7/05 by ned]

Some multi meters have a tacho function on them ?