RR Silver Spirit Speedometer calibration
I noted when the car was traveling at 70 MPH my Garmin SatNav said 62 MPH a nearly 13% error.
I wanted an accurate reading. The speedometer is electronic, it obtains pulses from the transducer UD24231 on the gearbox.
The difficult part was working out how many pulses per second for a given MPH. I researched the net - wheel size, final drive gear ratio, turns per mile but not knowing how many pulses come out of the transducer for one revolution I was stuck.
Eventually I deduced from the label on the rear of the speedo 8000 imp. that this represented 8000 pulses per mile an hour.
These are my calculations
8000 ÷ 60 ÷ 60 x 30mph = 66.6Hz
8000 ÷ 60 ÷ 60 x 50mph = 111Hz
8000 ÷ 60 ÷ 60 x 60mph = 133.3Hz
8000 ÷ 60 ÷ 60 x 70mph = 155.5Hz
8000 ÷ 60 ÷ 60 x 80mph = 177.7Hz
To bench test the speedo you need a signal source. I used this https://www.roomeqwizard.com/ Download the appropriate program and use the audio line output of your PC. In the program choose Generator icon, set to square wave and an RMS level dBFS of 0, enter the frequency in Hz. The signal needs to be boosted to drive the speedo. A single transistor amplifier diagram is shown on the right, it produces a good square wave to drive the speedo. To adjust the speedo there is a small hole covered by a label on the top, use an appropriate size insulated shaft screwdriver to adjust the preset.
After road testing, the speedo now shows the same MPH as the SatNav.
You may prefer to have your speedo read a few percent high, I suggest from the table above use 70mph 155.5Hz and set the speedometer MPH accordingly.