Bigger is not always better for filtering hash. Caps can exhibit what is known as "series inductance" and if the bothersome hash is high enough frequency, what might be more effective is LOWER value disc ceramic caps, with short leads.

This noise can be difficult if it's superinposed on a high-current conductor--like the main output lead. In that case one "help" is to hold the alternator--to battery lead fairly short and use the battery itself as a filter. Something amiss in the alternator can also cause excessive noise/ hash/ ripple---like open or shorted diodes, or open or shorted--or partly shorted--windings

Sometimes a "split ferrite" around the power lead to the tach can help

Welcome to the modern world of electronics--and a high rf noise floor.