It is possible. First, you need a datasheet for the Analog Automotive Tachometer. I am assuming it is a simple ammeter. <-- I may spell this one wrong... It is a needle meter. It work the same principle of a needle multimeter. The automotive store has those. I do have some needle meters here, and the only info for them is : At max current <-- needle at max value, what is the current ? The type I have are 10 mA ? type ? hum.. I need to check it out. Anyway, it work like : 0 current = no needle mouvement. at a certain current example : 10 mA - full needle mouvement, more than the current rated, it may damage the needle / internal coil.
If you have a datasheet for this particular meter, you may have more insight.
I was figuring I could just feed a signal into it but I am not sure what type of signal I would need to use?
Not that simple... The Arduino is Digital... 0 V = 0 and 5 V = 1 in and out, can take an analog signal 0 V to 5 V in, and a PWM signal , a digital signal , with a duty cycle. Therefore you need to "convert" a digital signal into a current signal.
Ok here the "trick" ---> Digitals ---> analog <-- Voltage ----> current. You got to use an external circuit. An Digital to Analog converter and a voltage to current converter. You have to use a few Op-Amps. <-- Operational Amplifier.
To use the PWM, you need : PWM --> Intergrator - > Buffer op-amp --> Voltage to current converter.
I hope that give you some insights. those are "Block" diagrams, you simply design each block to make this project to work.
I am sorry that it is not a simple answer.