Electronic speedo correction?

[wallisek]

Anyone know if it's possible to calibrate/adjust the speedometer on my 420? It's reading slow enough it's triggering my OCD and starting to make me slightly nuts. At 60 MPH my actual speed is 53 MPH, I verified via radar and my Garmin GPS. Running the stock tire/wheel combo it shipped with. Thoughts/ideas besides putting even smaller tires on it?  

[MV8]

If it doesn't have a speedometer cable, here is one way. I used one to adapt my fiero speedo to work with a chevy cobalt transmission and speed sensor: https://www.dakotadigital.com/index.cfm/page/ptype=results/category_id=287/mode=cat/cat287.htm

[CBuff]

I had the same problem. Even when I entered the ppu for a 420 with 13” staggered zzs set up it was still wrong. I tried to do the math to adjust based on gps vs what it was reading.  Ended up just adjust checking adjusting checking. And came up with this ppu that pretty spot on.  If you have different tires and wheels it will be different. But once you get comfortable entering the ppu you can check with against gps (I just used Waze or Google maps that reads out your speed ). 
** use caution when driving and looking at your phone. 
 

 

042081

 

 

here is also an automatic way to calibrate the Caerbont speedo:

Setting The Calibration – Inputting The Calibration Number / Pulses Per Unit Distance. There are two methods to setting the calibration: (i) Manually inputting the PPU number. (ii) Using ‘drive to set’ facility. 

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.

Drive To Set PPU: Set the LCD to display ‘DTS PPU’ as described. Press the reset button for 2-3 seconds, the LCD will display ‘∗00000.’ Drive exactly one mile. While driving, the odometer will count the number of pulses generated by the sender. Press the reset button for 2-3 seconds, the LCD will display ‘DONE.’ Within a few seconds the LCD will return to the ‘DTS PPU’ display. The setting is now complete. Switch off the ignition. 

IMPORTANT – When driving in ‘drive to set’ mode the speedometer will register but not accurately. The speedometer cannot be used on a public highway in this mode. 

[wallisek]

Thanks guys for the info. I'll try the procedure later tonight.

[7Westfield]

You can also use the highway mile markers to set it by mileage, that's how I did mine

Use a 10 mile run--at 1 mile they aren't that accurate, but at 10 it averages out

[Austin David]

@wallisek did you get it sorted?  there are a few methods, it's annoying but a good excuse to go for a ride.  I personally just ran at 70 mph indicated, used a phone GPS app, then did some light math to compute a new PPU.  Did that a couple times, it's perfect.

[wallisek]

I've been out of town but I did set the new PPU code, I found a reference image over on the Lotus 7 club forum. Mine was off by a couple thousand, input "039266" according to the chart for my tires (195/50/15). I'll be taking it out for a spin this weekend to verify/adjust.