Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Calibration Smiths Electronic Speedo

I have recently had to reset the calibration of my speedo because I received it back from Smiths set for miles instead of kilometres. The calibration is for the number of pulses per mile/km. In order to set it I made a proportional calculation based 1km = 0.62 miles which gave me 005210 (rounded up to the nearest 10).
Subsequent testing of the kilometres registered over a distance of 20 marked kilometres on the Superstrada showed that the distance measured was about 1.43% too much so I have now reduced the PPU to 005136. I will check this again over a long measured distance. The end result, when checked against GPS speeds gives me a result a fraction fast but acceptable. I am thinking that the best thing to do is to get it registering the correct distance and then leave it at that. Part of the issue is that the tyres I have fitted are 195/65 R15 and therefore cover more distance per revolution than the standard 195/60 R15 tyres.

The figure set by Smiths was 008380
Spanner Juggler for a Plus 4 used 008386
The figure mentioned by Black Adder for a KPH 2009 Roadster was 002555

It would be nice to get a collection of PPU numbers for different models. I would need: Year, Model, tyre size, and PPU number.

If I can get a full list I will publish it in a downloadable form. This information could be invaluable to someone who needs to set the speedo calibration.

In my case I held the button until after the needle returned to zero and it went straight into the manual PPU setting mode. It then flashed though all the digits in sequence and then exited the setting mode.

This is slightly different from the procedure described below extracted from the Smiths instructions

To read the PPU number follow the procedure below. If no changes are made the speedo will revert back to normal when the ignition is turned off or after about 1 minute.

Setting the Calibration – Inputting the Calibration Number / Pulses Per Unit Distance.

Setting the calibration mode
The calibration mode is selected by switching on the ignition while simultaneously pressing the trip reset button. The pointer will travel to full scale and return to zero.
If the button is released before the pointer returns to zero, the manual setting procedure will be selected and the LCD (odo.) will read ‘SET PPU’
If the button is released after the pointer returns to zero the ‘drive to set’ mode is selected and the LCD will show ‘DTS PPU.’
At this point, pressing the reset button momentarily will toggle between ‘SET PPU’ and DTS PPU’
Manually Inputting The PPU Number
Set the LCD to display ‘SET PPU’ as described above.
Press the reset button for 2-3 seconds, the LCD will show the calibration number currently set. Each digit
within the calibration number will flash in turn for approximately 2 seconds. When a digit is flashing, each depression of the reset button will increment the digit by one. Once the last (right-most) digit has been set/reset, the whole number will flash. Press the reset button and the LCD will display ‘DONE.’
After 3 seconds the LCD will display ‘SET PPU,’ the setting is now complete. Switch off the ignition.

I think my speedo under-reads a bit, is the display on a sat nav good enough to check it.

Under reads? You mean that when the sat nav shows 80 your speedo is showing 75? If that is the case it is a bit dangerous to your wallet

Speedos normally read a little fast, even as much as 10% sometimes but the normal is within 5%. My feeling is that even if the distance registered is 100% accurate the speedos are still set to read a fraction fast. Perhaps this does not apply to a digital speedo that shows the actual numbers.

In Italy the highspeed roads, Autostrada and Superstrada, have accurate markers every 10th of a kilometre and a large marker every kilometre. It makes it relatively easy to test the digital registration of these instruments. I found a layby which was very close to one such marker and zeroed the trip metre. After 20 km I pulled into another layby and compared the actual distance to that registered in the speedo and adjusted the speedo accordingly.

A sat nav will give you a good guide but I don't think it is accurate enough to set a speedo and, with all the care in the world, it is impossible to hold a really steady speed.

There must be a lot of different pre-settings on different cars.
In an old thread from 23.3.15 / ..Calibrations / Maint.&Servicing / there is a posting from Andre proposing 022800 for more accurate speed.
I changed from the original setting 021200 to 022800 and now the speedo shows quite realistic figures. My 4/4 was built in 2011; now tyres dimension 185/70R15.
Before the resetting the speedo was more than 15km faster than GPS at 100kmh

is that the same setting for mph speedo ?

If your innitial setting shows the same numbers ist should work.
If MPH has different basic settings it will not work

my experience is based on the world of metric systems

Useful post Peter, thank you

The mph and kph speedos count pulses per unit of distance so in theory to convert the PPU of a kph speedo to the PPU for a mph speedo you would have to multiply the PPU by 1.60934, and to convert from mph to kph you would multiply by 0.621371

An issue I have with my (second) speedo is that it reads less speed the faster I drive. I.e. if 60 km/h is quite reading ok, 110 km/h is in reality about 116 km/h.

This behave keeps independent of the basic distance adjusting setting. So what is the better way to do, set the "right" distance counter or be sure not to drive too fast caused by a speed reading which is too low.
On the other hand I would not like to put too much (undriven) miles or kilomtres on the counter.

I'm still trying to work out my approach to this issue. I have a feeling that partly because of the low pressures used in the Morgans the tyres effective rolling circumference may increase at higher speeds. Rather than trying to adjust the number to represent the actual speed I am thinking the best thing to do is to adjust the number to get accurate distance measurement at a constant speed. It should be possible to get it very accurate using the measured distances on the autostrada and compare it with the distance measured by the trip metre (accurate to 1 tenth of a kilometre) over 20 km this is a 0.5% accuracy, probably even closer as it would always read down to the last tenth passed so one should always round any result up.

Worth mentioning that if the odometer is reading less than the actual distance travelled you need to reduce the PPU, and if it is reading more than the actual distance travelled you need to decrease the PPU.

If you more more concerned about speed accuracy and the speedometer is reading fast you need to increase the PPU and if it is reading slow you need to decrease the PPU.

By the way don't forget that this number will be different according to the diff ratio and the tyre size, so the PPU for a Roadster would not apply to a 4/4.