Speedometer Calibration RPM guage idea.

[Kevin in NJ]

This is for the people who have an understanding of electronics.

To cal the speedo you need an accurate RPM. There are articles at Bob Johnsons website, basically 1000RPM is 60 MPH. There is a magnetic shunt that is moved inside the speedo to adjust the speed.

I am looking for a quick temp way to accurately do 1000 RPM. As cheap as possible

Having an accurate speed source is the next problem and that means having an accurate RPM meter.

I did some searching around and found this program:

Rev Counter

It is a modified Guitar tuner that uses the microphone port on a PC or a Laptop to see a ground.

So now all we need to do is ground out the sound port.
One thing I ran across was taking the IR transmitter and reciever pair from an old ball mouse. The problem with this is making up a bunch of stuff for an IR interuppter. I am sure it could work, but it was quickly becoming a lot of little pieces needed to be set up right.
IR RPM measuring

A friend at work bought some Hall effect sensors to do some model train automation. He gave me a TT Electronics OH090U Hall Effect Sensor. It is an open collector device that just needs some power and a nice strong magnet. They are cheap at only a few dollars each. I wired power up to pins 1 and 2 and connected the wire from the Mic port to pin 3.
I had a problem with too weak a magnet and my little power supply causing noise. A better magnet and a 9 volt battery and my laptop was giving me RPM.

Now that I have a working RPM meter I can set up to drive the speedo with an old cable.

For those less tech inclined...

Do a search on Ebay for a laser RPM meter, they are about $16.

[Fullraceflathead]

These are all good ways to check the accuracy of the model A Speedometer.
I think that the easiest and most accurate way now a days though is to use the GPS reading that's on most cellphones today. The Ap. was free on my iphone. If you don't have a cellphone with this Ap. ask your Friend!

[Mike V. Florida]

Stop watch and a measured mile;
http://web-cars.com/math/speedometer.html

[heneste]

I agree with the gps option as you can tell your speed very easily. They can run ok on the 6 volt system also.

One question about the relation of rpm to speed. Would it not depend on gearing in rear end as well as tire size (if modified from stock)?

[Kevin in NJ]

The gps is ok at figuring out speed. What good is knowing you speed is wrong if you can't fix it?

BUT

You need to have the speedo apart on the bench to make it right. On the bench you run the input at 1000 rpm and then you can move the magnetic shunt to get the correct speed showing. Then you do not need a GPS to tell you what speed you are really going.

Quite frankly, My brother and I used this thing called a watch to find out his speedo was running 5 MPH low timing on mile markers. Got kind of boring driving from NJ to WI so we checked it out well.
So all the time the car was run 55 to 60 we were really going 60 to 65 MPH (no- this is not an April fools joke). Keep in mind his car is fairly stock.

[Mark in MT]

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You can tell if your car's speedometer cable is being driven at the right rate if the odometer adds up the miles correctly. Take it out for a drive and note the mileage on the odometer at a certain mile post, drive 10 miles and see how many the odometer added up. The percentage difference is how far off your drive system is. The factors that contribute to this error are tire size, rear end ratio, and driving gear tooth number on the torque tube. If the odometer is off, the speedometer should be off the same percentage in the same direction. In that case, you need to alter one of the variables above to change the cable speed.

If the odometer is correct and the speedometer is off, then the speedometer itself needs calibration.

I have a couple times, "custom calibrated" a speedometer to make the speedometer read differently from the odometer, as the customer wanted accurate speed, and didn't care about the miles.

[Bill Stipe]

Kevin, you need one of these for your 1000 RPMs! just chuck a piece of speedo cable in the chuck clamp speedo on the table and program in spindle RPM 1000 and hit cycle start@ LOL
Its a little spendy for only checking speedos.

[dlshady]

Yeah Bill, I'm afraid you'd have to rebuild a LOT of speedometers to justify that to the wife!

[Mike V. Florida]

Look what I found should get you real close;

1000 rpm motors