A distributor tester is a device used to check the advance curve of either a vacuum or mechanical distributor and so help the operator modify the curve to suit a particular engine.

With progress in engine technology and the gradual obsolescence of distributors, these devices, once common in tuning shops, have become rare. However, they still perform a valuable service for owners of classic cars and hot rods. The latter achieved much of their extra power output through modification of their ignition advance curves thanks to checks carried out on distributor testers, whereas today the advance curves often go unchecked.

Checking the advance curve on a distributor tester is a lot less stressful than on an engine. The tester can check the distributor at 6,000 engine rpm, or more, without involving the engine at all. (Note: The readings supplied by a distributor tester need to be doubled as the distributor turns at half the engine speed.)