When traveling at high speeds you have the best of all engine cooling conditions; a high flow rate of coolant through your system and a high rate of cool air passing over your heat exchanger. 

Occasionally we have an inquiry from someone who describes overheating at high speed conditions.  We walk through the possible causes:

• Is the temperature reading confirmed in multiple way to rule out a gauge failure
• Is the return hose or suction has not collapsed due to greater draw at higher rpm choking flow?
• Is the flow restriction caused by too few or too small a tube
• Is the radiator core clogged with sediment or lined with scale which impedes heat exchange
• Is the water passage in the jacket blocked
• Is the pump impeller cavitating
• Are there any obstacles to the air flow in front of the radiator?
• Grill opening, winches and jacks etc.
• External transmission coolers
• Turbo air charger
• Air conditioner condensers
• Pusher fan
• Is the fan shroud depth permitting air to escape
• Is the impeller intact and secure on the shaft?
• Is air exiting the engine compartment?
• Is the radiator surface area and face area adequate for the engine demands?
• Is the thermostat functioning or stuck or restricted?

If we can rule these out then we would suggest they look at the engine and why its generating the excessive heat.