Did the new sparkplugs have:

longer projected nose
sharper tip electrodes
bigger gap?

the above 3 design features "usually" result in slightly faster combustion and bring the stock degrees of ignition advance closer to the ideal setting of " mean best torque" or MBT

this usually results in a little bit better fuel economy, torque and power all at the same time

most Chrysler engines seem to be retarded from MBT
by anywhere from 6 to 10 degrees or so
for safety sake to reduce warranty claims for detonation damage
when an unlucky tank of less than 85 octane gasoline is bought
on hot dry summer days

this means faster burn sparkplug features
can gain benefits
if you are willing to to reduce the factor of safety
against detonation

the rough rule of thumb is
10 degrees of retard from MBT timing
results in 5 % less torque at full throttle
but allows burning gasoline
5 Octane points lower before audible pinging

Fuel Economy-wise
it is always better to wipe out degrees of retard before MBT
with better burn sparkplugs ( or cyl head chamber design)
than by simply advancing the ignition setting

when the burning begins before the piston reaches top dead center it creates "negative work" that the other cylinders have to overcome

ideal cylinder heads and sparkplugs combos
would provide MBT
at 0 degrees top dead center

one guy doing a Masters degree in Engineering a MIT
added a second sparkplug to the side valve CFR engine cyl head
and found gains in
fuel economy,
torque and
power
after the degrees of ignition advance
were reduced to a new MBT setting.

He decided to do his thesis
on how many sparkplugs
would be needed to get best results.

He got up to 13 sparkplugs
yet found there were still measurable gains from 12,
but now there was no more room
on top the side valve cylinder head for another threaded hole.