How i recalibrated my Speedo

[450sl]

For those interrested this is how i recalibrated my Speedo:

Took out for a test drive with my Gps and drove up to 100Km/h onto the Gps.
Noted the exact Gps Speed and Speedoreading.

Connected the Speedo by a spare cable onto a frequency-controlled motorspindle , and adjusted speed to the earlier noted speedoreading and stored it into the controller.
Removed the glass and pointer from the speedo and put it on the teststand again.

Started spindle with preset speed and pushed back pointer on its 100Km/h mark.

I guess if you dont have the tools ,but are not faint hearted you could do this whilst driving.

Mark

[kampala]

I love it ... can you advise if it's now accurate at other speeds as well? say 50kph?  or 130kph. 

what speed was the speedo showing originally when gps was showing 100kph.

My speedo is accurate but find your solution interesting ... appreciate you posting this ...

[jameshoward]

I need to adjust my speedo following a rear diff change so am interested in speedo solutions. Problem is, I don't understand what you've done. What is a 'frequency controlled motor spindle' for the uneducated like me?

[George Des]

Yeah, I'm not convinced this is a solution so I'd also be interested to see how it works out over a range of speeds. The odometer is geared and my guess is this calibration is determined through different gearing

[georgem]

So, in summary I think it goes like this....
Assume out on the road, the speedo read 110 and the GPS said 100
Back in the garage, take out the speedo and remove the glass from the front.
Connect a spare speedo cable to the speedo and  the other end to an electric motor with a variable speed controller (frequency controlled motorspindle)
Run the motor until the speedo reads 110, then push the needle back to 100
Reassemble and reinstall etc etc

All good so far but as you say George I suspect that you now have a very accurate speedo for 100kph. As I understand it, speedo error is normally recognised as a %, ie originally the speedo was reading  (say) 10% high and this was probably across all speeds. Because its a mechanical device, with gears and magnets, even though the needle has been reset for 100kph, it is still 10% innacurate - so by moving the needle, at low speeds, where it used to read 33 instead of 30, now it will read 23 instead of 30 (approx).

I suggest that the GPS/speedo check needs to be done at 10kph/mph increments up to 100/60. From this calculate what the % error is - ie if it is reading  slow or high. The variation, whatever it is will probably be consistant.  PLUS next time you do a reasonable length drive, say 50kph, check the trip meter for accuracy - you can use the GPS to help with this too. These two bits of info are useful.

If both the speedo and trip meter are reading the same % variation, then start thinking tyre/wheel size, if the trip is accurate and the speedo is out, take it to a instrumnet repairer and they can fiddle with the magnets etc to adjust the speedo variation.

cheers

[garymand]

Good point on the trip counter.  It will be still sunk to the gearing and wheel diameter.  He has only zero'd the inaccuracy at 100km/hr.  The dial reading accuracy will increase both up and down from there.  Not a bad trick if that is good enough to make you happy.

[450sl]

Odometerreadings are not changed , my speedoreadings where 20% off due to getrag 5speed conversion.
cannot tell yet if speedo is accurate all the the range yet due to beiing away from the car for some time will let
you know.
as there seems no other way to calibrate this must be the way pros do it.
Only thing that can be altered is the pointers returnspring preload this definately has its effect on accuracy.

[georgem]

Ahhhh...............the Getrag moves the goal posts so I await with interest your test next time you have a chance to drive it

[garymand]

True the odometer reads are not changed, the y are just as inaccurate as before.  You only moved the pointer.   In the world of Calibration, this is called a single point calibration.  It is only good at that point for the pointer.  As your speed changes away from that point the speed error will grow.  Before you had no error at zero and noticable error at 100km.  Now you have no error at 100km but the same rate of error that is greatest at zero and 200km on the dial.  The counter is still as inaccurate at any rate of speed.

[GGR]

With age the magnet effect of the speedo may be altered inducing some inaccuracies in speed reading. Odometer is not affected. Recalibrating involves bringing back the way the magnet was acting on the speedo (I don't know how specialized shops do it).

In the case of speedo gear drive ratio change due to a different transmission, or in case the of a rear end swap (or both in my case), then both odometer and speedometer are off by the same %. The easiest and cheapest way to address the problem is to fit a ratio adaptor in series on the speedo cable. One needs to determine the % by which the ratio should be changed and then send a speedo cable to a specialized shop which will put the right gearing in a speedo adaptor and modify the cable to fit the adaptor in series on the cable. This is the way I did it and I'm happy with the result.

[RonB]

The most accurate way is to have  Hollywood Speedo set it for you - they have the gearing. Do a nice job - had mine done recently.

They do all the time, especially with bikes when they change tire & wheel size.

When they do it they take in account the tire size. You have to do a little work for them when you send it in.

[Rolf-Dieter ✝︎]

Mine is out due to tire size. I will leave it as is until I changes the tires to stock size, meanwhile I use the GPS readout.

I saw a iPhone application that shows the exact speed in digital readout, then we don't need all that going along peaceful country roads do we now

[450sl]

First Sunny day  did 250 km trip (used  Garmin Zumo550 AND  Zumo 590 Gps)  check of the Speedo 

Speedo readings       Gps readings

45 km/h                   33 km/h
55 km/h                   45 km/h
65 km/h                   59 km/h
72 km/h                   70 km/h
80 km/h  -120 kh/h    Spot On
120 km/h                 not checked

Odometer readings are approx  15% lower then actual  Gps Odo  (approx because of af few very small hitches in trackreadings)

Accurate enough ?  for me i am a happychappy

[garymand]

If you had set it for 70 it would look like this:

Speedo readings       Gps readings

45 km/h                   39 km/h
55 km/h                   50 km/h
65 km/h                   61 km/h
70 km/h                   72 km/h
75 km/h                   75 km/h
80 km/h                   78 km/h

It is not a linear progression.  The farther from your set point the more inaccurate it is and vise-versa.

Like you say, its probably close enough