Thanks for the replies!
Eric, could you explain the circuit you have drawn a little more for me please? I did basic electronics about 8 years ago and haven't really used it since! Some of it I understand, Vin is my voltage I want to modify, R1 & R2 are of equal value as I do not need any gain on the circuit and Vout is the modified voltage going to the gauge. What is the purpose of R3 and the LT1009, does the + pin of the op-amp they connect to need to be at 2.5V? If so, I have another 5vdc (regulated) supply, I could use another potential divider circuit on this to achieve 2.5? Is it possible to use the car battery to power the op-amp, bearing in mind this will be between 11 and 14v, but should be 13+ so long as the engine is running (which is when the fuel gauge is important!)
Many thanks!
Andy