My 49 Special (standard, no overdrive) is much much happier running just a bit under that, at about 55 mph, true speed via Garmin Nuvi GPS. It tends to run hotter above that, even with a rebuild a few years ago, and everything boiled out. I so wish it had an overdrive. I stay off 4 lanes here if possible, when the speed limit here is a nominal 100kph (62 mph), and I can't even maintain that in mid-summer without it running warm. It would be better if you could install an overdrive and keep that r/e ratio, instead of changing it to a 4.56 which they came with. Then you would have a car that could maintain modern highway speeds at a more sane RPM. The Continentals in all applications preferred lower than 3k RPM, according to my tractor sources.