Electric fan won't cool at highway speeds

TylerW said
I bet there is an undersized or clogged radiator behind all this.

The thermostat is open at anything above it's rated temperature and if the temp keeps going up past that you have problems ELSEWHERE.

I would add that it is possible the cooling passages in the block are full up to the freeze plugs with Scale...and coolant flow is taxed as well....

What he said....^^^