At 70 MPH my Tach indicates 3400 RPM. My 123 distributer shows 3000 RPM.
Which one is correct? The 123 is a bluetooth device.
Thanks Brian

It's going to depend completely on the size of your tires

Your tach could easily be that far off, I don't know how accurate the 123 is.

You could use an engine rpm calculator, if you have a stock trans it's easy enough to look up the final gear and R&P ratios. Not perfect because tires wear, but should be close enough.

Brian - both devices are counting pulses from the engine rotation, so gearing and wheel/tire size aside - what you're really asking is which engine RPM reading do I believe?

So assuming you have a stock VDO tach, it's internals rely on analog electronics and magnetic components to waggle a needle at the right angle when it sees some electrical pulses. All those components are around 50 years old and MAY not be accurate any more.

I assume - but I don't know, as I haven't seen the circuitry - that the 123 is counting incoming pulses and comparing them against a very stable, accurate timing reference, then working out the RPM with microcontroller code & math.

So personally - I'd believe what the 123 is telling me. But don't ditch the VDO gauge - its a thing of beauty and is trying its best.

- Tony

Thanks
I tend to believe the 123 dizzy. the VDO tach is long in the tooth!

You could always use a timing light with RPM display. You could check your tach and your timing in one swoop.

X2

Once you know your tire height, plug the numbers in here...

RPM Calculator


...and you'll know which is correct.


If you don't know your tire height, plug the size in here to get it....

Tire size calculator

my tach is almost that far off too. what does your multi meter or timing light say ?

Use a speedometer app on a smartphone... then compare.

Here's a gear/rpm calculator I developed years ago that is more specific to our cars:

Gear Chart Calculator

Use the "Help" button for a detailed explanation of how to use the chart.

see a lot of cars here on 4 bolt wheels with undersize profile tyres,looks cool but screws up the speedo and burns up fuel.
Don't trust an old tach ,my one was reading 5700 and the rev limiter rotor was ticking off @ 6400rpm as designed.
I have had shocking luck with VDO tachs so now I going to transplant the internals with new (modern) circuit board.

My gps reads 72 MPH and the speedo indicates 70 MPH. So they are pretty close.

Believe the 123.
Though it can be apples and oranges depending on tires, at 70MPH most 914s are turning ~3K in 5th.

Zach

Believe the 123 for accurate engine RPM, but that distributor has zero clue what speed you're going.

A GPS is the only instrument readily available that tells you the actual sdpeed you're going.

It's safe to presume you have stock gearing, and it's easy enough to find out the correct tire diameter. From there it's an easy math problem to see what speed you should be going.

Then compare that to the GPS.

Poof: done.

@pvollma - thanks for that calculator. I was pretty sure that in the 90's my stock 73 1.7 would cruise at 70 at 3000 and this confirms it. It also confirms that my 74 with 195/55/15's needs another 250ish rpm to do 70. Interesting.

I am running on 205 60 15 tires, when I was clocked by one of those signs that tell you what speed you are going ie. entering a construction or school Zone, my speedometer was reading really close to what that radar said.

Ian Stott
Moncton
Canada