After 15-20 minutes driving over 45 mph Temp guage climbs to peg

U gutta leak or cap is bad. Fan motor may b weak or grill is dirty

air lock in the cooling system get a plastic bottle cut the bottom off turn upside down put on radiator filler and run engine cut full off water wait until all air bubbles stop then remove and put cap on see how that goes

Does the gauge climb if the sensor is disconnected ?

Yes, if the Gauge continues to climb it is either the sensor or the Gauge

First question is what constitutes "overheating".
Since the car has been parked for years, the thermostat could be sticking closed.
The cooling system could have air pockets and need to be bled.
The water pump may be corroded inside and not pumping enough volume.
If the gauge goes above half way, the fans should start up. But half way should be more or less normal, not hot yet.
I would look at flow problems first, beginning with removing the thermostat and driving it without one for a few days. Also check and see if there is a process to bleed the cooling system of air.

Any chance of a head gasket leak? Is the coolant level still full? These were known for head gasket issues. Coolant is a closed system and level will not drop unless there is a leak. Make sure there is no air in the system. Replace the radiator cap to make sure that it is operating properly.

Check if your coolant decreases. If it does it may be a leak on the system. Check for the radiator cap that may need replacing and the radiator hoses for loose or bad hose that needs replacing. Check the water pump found inside the engine. Because of the car's age, it may need replacing.

check for a leaking intake or head gasket

Most systems work this way: temp sensor on engine sends a signal to the computer which in turn sends signal to blower relay that then sends signal to the fan.
Any problem along the way, and the fan won't come on.

Q: Temp starts to climb by how much? If the vehicle interior is hot"over 90" , keep in mind the heat in the vehicle is being absorbed by the a/c and carrued out to the "condensor" in front of the engine radiator. Engine temp should be expected to climb somewhat. As the interior cools down the less heat is taken to the font and forced through the engine radiator. It should peak out ,then staer deopping or stabilize. On initial entry of vehicle on hot days it would help to open the windows for a mile or so to purge excessive heat from passenger compartmentw/AC OFF. OUTSIDE CAN BE 90 AND THE INTERIOR OF A CLOSED UP VEHICLE CAN CLIMB TO 120 DEGREES + mID 80'S IS HANDLED WITH GREATER EASE FOR A COMBINATION OF TECHNICAL REASONS OF WHICH WOULD BORE YOU.It sounds like all is well so be cool(pun) and don't waist money chasing a ghost. Happy trails. Oh, The auxillary fan is good to speeds up to 35 mph. Above 35 the forced air from the vehcle speed does the fans job.