It won't work on a carburetted car. The MPGuino needs a Vehicle Speed Signal input, which many carburetted cars do not have, and it needs a fuel injection pulse signal. No carburetted cars have that.
You'd probably be better using a vacuum gauge. If you really need
some MPG reading, and don't need it to be very accurate, you can add some sensors to your car (a manifold vacuum sensor, for one, and an RPM sensor on a drive axle or some such, and possibly more) and estimate the fuel consumption from that. But it will be a fairly imprecise estimate...
-soD