I would have some hesitation adding to the VDO guages with Smiths (Caerbont). I have found them, Caerbont, generally unresponsive without persistent emails and telephone calls. I had them change the colour of my speedo and tacho faces and make replacement guages for the 3-in-1 instrument and a new clock. After 3 attempts they still could not get them the fuel, and temperature guages reading correctly and I have had to compensate using resistors on a trial and error basis in order to get "near enough" readings at critical points (ie, running temperature and near empty fuel tank). The 3 instruments were supplied with LED lighting of different colour of white and the clock has an incandescent bulb. I have spent a lot of time with LED replacements for the speedo, tacho and clock to get a near match.
All in all a lot of time, expense, and faffing about.
I think that if the original instruments are VDO I would be only thinking of adding to them using VDO from the same series.

The one thing I will say in favour of Caerbont is that they will produce custom guages to your design at a reasonable cost but why they have difficulty programming them correctly I can not imagine. Unfortunately they did not allow for an external interface to change the calibration and the guages have to be dismantled to adjust the calibration table.

It is also true to say that guages function on very low currents and poor connections may have some bearing on results. I have been told that in order to get correct fuel gauge readings an London Taxis they had to run an earth wire direct from the sender to the battery.

Peter

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