Hi,

If the wire from the sender is frayed and if the core conductor inside the wire can touch a bare spot of metal it will read to the top.
If it can touch another wire in the harness as to get a grounding circuit or energized wire the gauge will read to the top.
Some cars up to around 1987 suffered engine harness issues mainly from either biodegradable insulation or spray on engine cleaners not getting rinsed out of wiring harnesses.
Both are valid excuses to cause your issue.

As far as the fuel gauge that could be a calibration error from the sensors arm inside the tank.
That sensor also uses resistance to limit the current to the fuel gauge.

Now, since both gauges are showing a high end readings, there is a voltage regulator on the instrument panel itself, that is supposed to limit voltage/current across the two gauges.
This is to be about Ten volts and it pluses on and off all the time the key is on.
You might look for that going out to the sender.
The newer instruments clusters with electronic speedometers, not cable driven, have solid state regulators and as far as I know they don’t pulse slowly enough to be read with a voltmeter.
In either case, you need a constant voltage that’s under the battery’s supply voltage. It needed to provide accurate readings on both gauges.

If both gauges are substantially off in that their readings are more than they should, be suspicious of the voltage regulator being faulty. They are mechanically prone to this but have been reliably used for many many years.
The gauges then receive or use a current from these regulators, through a sender unit to heat inside a bimetallic lever that moves the needle in both of these gauges.

As far as analog gauges are concerned, it’s the system is still used today in many things not so hi tech.
The early Volkswagen Bug vehicles use a light bulb to limit the current.
These cars were notable for being made in expensively and worked on a lot.
If the light bulb burned out the fuel gauge quit.
Luckily, Not too hard to get to though.
Bicycles and Harley Davidsons were career starters for a many mechanics of my day.
Or in most cases, by anyone needing more skills in something important.
Old machines are like textbooks from our ancestors that are not in libraries or downloads.
Working and studying on them teaches us more than just how to read but the reasoning behind them.

As far as temperature gauges have evolved you look out over the radiator to see a real mechanical gauge out there. They were using a Bourdon tube, that’s still in our pressure gauges today.
These Do Not require batteries.
I will not say that digital is an improvement in reliable technology, if its blank.
Fast Thumbs on phones, I’m sorry, but I don’t see the fruition of our minds coming from some games.
It’s More like dumb-ing down.

So If both didn’t work look there. An excessive draw on the regulator can affect the fuel gauge, one would think?

I hope I gave you some ideas of what you are “looking at working” and can see what might correct it.

Phil