Slim to none. The biggest improvements they deliver come in the form of quicker and easier starts, along with set it and forget it tuning. If you add a hamilton efi kit along with it, you might gain a couple mpg that way. Take a gander thru the injection sub-forum. Bill hamilton offers turn key kits thru ihon, plus he also explains how to do it yourself with boneyard parts as a lower cost option. He also details another low cost diy option for electronic ignition which involves minor modification of your Holley distributor combined with specific parts from a Ford duraspark dizz. Honestly the ceiling for gaining mpg is fairly low on these pigs. Vehicle weight, aerodynamics of a brick paver, 1950's engine technology, and gummint-forced consumption of corngazz are all conspiring against solid fuel economy. Maintaining an optimum state of tune no matter what components you are using, combined with responsible driving habits, along with an attitude of just appreciating these rigs for what they are will yield the best results and enjoyment over the long run.