DO NOT use a 30 amp ammeter on this car. It has an alternator, 30 amp ammeters are designed for the lower current produced by a dynamo. The alternator provides enough current to fry this gauge, when it does nothing on your car will work and there may be a fire. When fitting an ammeter consider that all the power in the car flows through it and if it fails at any time you're stuck. You say you want it to show the current flow but not a constant, well you can't choose what it shows. It shows the current in the wire you splice it into. Basically all they show you is whether current is flowing to or from the battery which was very important with a dynamo (which only charge above a certain speed) but is almost useless information with an alternator. As long as the no-charge lamp isn't lit and the alternator can provide more than the accessory load the battery is charging because they produce the same voltage at all speeds once started. That's why cars don't have ammeters any more, they just aren't needed and most people wouldn't know what it was showing. A voltmeter is far more useful on a car with an alternator. If you want to fit one find a 60 amp, if you have a beefier alternator than that you can't have an ammeter from a range of car gauges because nobody makes one with high enough capacity. If you do fit it, make sure it's safe and bear in mind the possibility of failure.
Just to add to Dan's Post : There are two types of ammeter available , namely ; in-line type and Shunt type.
With the in line type mentioned above current flows through the ammeter providing a direct measurement. The Shunt type ammeter uses a metal bar which current passes through and the "ammeter" is actually a voltmeter measuring the volt drop across the metal bar , with the guage reading calibrated in amps.
Shunt type ammeters can be used for measuring much larger current.
A "stalled" starter motor , for example trying to operate with the engine prevented from rotating
is quite capable of drawing a few hundred amps current.
