1968 Barracuda with internal gauge régulator

For others, it is easier to use an aftermarket adjustable regulator. Search "voltage limiter" w/ "Plymouth" on ebay. $30 to $55, the later being RTE. Many photos and links showing exactly how to wire to replace the thermal reg inside the fuel gage (until ~1967 clusters, to 1973 if Rallye cluster i.e. tachometer).