Faster I go the cooler it gets!
 

Not really! I took the 37 on a 600 mile round trip this weekend. High 90's during the drive, running the A/C the whole time, cruising 70 to 75 the whole way. Temp stayed around 210, got a little hotter going through towns along the way, 220 tops. It would cool back down to 200-210 at speed. On the way back I took the toll road, speed limit 80 mph. I ran it up to 2500 rpm which was about 88 mph. After a few miles I checked the temp gauge and it was dead on 190! It didn't budge until after 30 miles I got back on the interstate and slowed down to 70-75, It then went back to 200-210! Any thoughts? I'm thinking maybe a smaller diameter wp pulley to increase flow at lower rpm's.

The smaller pulley should certainly help. Have you checked the rest of the system? Shroud, fan, waterpump clearance.

Usually cooler at speed indicates insufficient air flow at lower speeds. I'd look to the fan, fan clutch, shroud, various proper baffles and seals in place (specially with AC).

All this assumes the radiator is up to snuff and the additional air flow at speed isn't just helping out a marginal radiator.

Thanks for the reply, this is my 37 with a fairly stout 462. Car has been together for a while now, I've put about 30k miles on in the last 7 or 8 years. Only thing new is the vintage air installed in 2018. Pump clearanced, aluminum radiator, 2 spd electric fan and shroud assembly. never ran above 190 before the A/C. Not a lot of area on a 37 for airflow but I feel I'm maximizing whats there. I was just surprised that it cooled down 10 degrees running 88 mph vs 70 mph!

Moneypit...I find that surprising too...thus the questions about fan , waterpump etc. It sounds like you have all that worked out. You said you have an aluminum radiator. Is it a four row or is it a two row with 1.25 inch tubes?

You mentioned 2 speed electric fan. Some have found the fans actually restrict the air flow through them if they're in front.

It's a 2 row Superior, can't remember the tube size. Cooling Components "puller fan" comes on low speed with ignition, thermo switch kicks it to high speed at 190 degrees. The car is not over heating, made the 600 mile trip without a hitch. I was actually trying to test the limits of the cooling system by running 90 mph for 30 miles on a July afternoon in Texas. I was just surprised that it that it ran 15 to 20 degrees cooler!

I guess the addition of the AC condenser reduced air flow enough to cause what you are seeing. Electric fans just aren't going to move the amount of air that a big factory fan will, specially with the factory shrouds designed to work with them.

The factory went to considerable lengths to up the cooling efficiency on AC equipped cars.

You might be able to just add a few baffles or seals to make sure at low speed it's not recirculating under hood air.

If it's running above the thermostat setting it's always a bit scary, that means the cooling system is maxed out under those conditions.

This situation is highly related to the surface area of the grill of your Rod. Have seen this same thing on other 30's cars . If you have a shroud, that can help, but you are running a ram air radiator at 80+ !!

My coolant pump is overdriven 23%, factory pulleys. It absolutely has a sweet spot where maximum heat is removed, 2300 engine RPM. 2829 pump speed. Do you know your pump speed ratio?

your numbers are VERY unique to YOUR vehicle cooling capacity, Calorie burn and Area of the heat exchanger tha'ts used ( radiator) .Your "sweet spot" Also has a lot to do with how much air you get across your heat exchanger, and that airs temperature.
I run a A/C pump pulley set up, so my stock pump is slightly overdriven, but it is only a circulator.
I run mine with a AC Delco 170 degree ( static) T-stat.
You might like a 160 .
Sounds like your car is very close to dialed in as best as you can with the Grille.