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It would be more accurate to say 'fuel waste' being less with electronic, than with mechanical... and as I noted before, if everything's in tip-top shape, there'll be no noticeable difference... but being mechanical, and subject to wear and environmental degradation to which the electronic trigger and amplification system are not, the electronic will waste less wholly based on the mechanical's state-of-health.
Much of the fuel waste occurs during the 'wet' portion of startup... when liquid droplets get drawn in through a cold manifold. A wet plug doesn't develop an arc- the liquid fuel, and moisture, and oil short it out. When this happens, It takes a few cycles to 'blow' it clear, and if the choke is still pulled (to make other cylinders fire), then it'll keep flooding until the engine's spinning fast enough to blow it clear.
Electronic ignition floods just like mechanical, however, it's not apt to START missing on a result of improper firing. There's no contact points to become fouled with frost or corrosion, nor does it require a condenser, so it's less likely to foul, and much quicker to fire THROUGH a foul, hence, the engine starts 'wet', and gets off the choke sooner.
Realize, that the intake path of a carbeurated engine is 'wet'... manifold vacuum is generated by airflow through the carbeurator venturi, and it's drawing fuel up through the manifold, which because of ambient conditions AND combined gas law, will remain cold until such time that some external heat source warms it. The heat source, is the exhaust manifold junction point. This junction conducts enough heat from the exhaust side to the intake side, to minimize intake and carb frost, as well as helps vaporize the fuel droplets so they'll burn.
So the parry, is that a misfiring engine will not generate exhaust heat, and without exhaust heat, the misfiring engine will not clear and fire. By establishing a good fire from the outset (getting it touched off, and then off the choke), you'll get good atomization, much quicker warmup (out of the 'wet' stage), and much less fuel used.
When running HOT, you have the opposite condition- very well atomized fuel, and when the carbeurator is in an 'economizing' state, the mixture will be pretty lean... it takes a hot spark to touch off a lean mixture... especially in a slow-turning low- to moderate compression engine. The electronic trigger system does that... because it doesn't need 'point dwell' in order to fully saturate the ignition coil at every event. Electronic trigger systems don't suffer 'bounce', or contact contamination.
So the fuel economy benefit is that when you put a set in an older machine that HAS seen it's fair share of fencerows, it gets back what it USED to have, but without having to babysit the gap and dwell, no need to keep the contacts clean. If your mechanical system was spot-on-perfect, it wouldn't matter, but the maintenance to keep mechanicals that way, is substantially greater.
Nice thing about the Pertronix kit, is that it takes 5 minutes to slap it in and wire it up, and no messing around to get it working.
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