How about you send me the 6 volt fuel sender and i will send you a 12 volt one ?

Louis,
as mentioned previously, I'd reckon that the resistance of the gauge plus the variable resistance of the sender also need to be taken into account.

Someone has already put these values up on the internet, however I'll leave you to do the calculations, as I'm rather busy working out VAT and tax for HMRC at the moment.

The 2CV fuel gauge resistance across the terminals is 140 ohms and the resistance through my pattern replacement 2CV tank sender is 338 ohms at empty and 10 ohms at full.

On a quick and dirty assessment, the lowest resistance on that 'V out' leg would be 150 ohms.
Call that R3, so the combined resistance of R2 and R3 would then be 75 ohms.

At which point, that voltage divider circuit would no longer be halving the input voltage...

ken

I'll do some thinking today, maybe even play about on a simulator but to my mind the out put resistance shouldn't have too much of a bearing on output, but I'm willing to be proven wrong!

Edit on that last one I'm clearly tired! The resistance of the fuel gauge and sender must be the same as that of the 'R2' resistor but they're variable and in parallel... Should be interesting!

I guess like this the gauge would work backwards? Not sure how the fuel gauge works.

Made this work with 10 Ohm resistors, I guess the differential is enough that the voltage change caused by the fuel sender is negligible.