The sidewall says 100 psi and everyone tells me that is far too much, what pressure should I actually run?

How to arrive at your number, practically.

Start from a chart. Several tyre manufacturers publish tables of pressure against rider weight and tyre width, and there are calculators that do the same. Put your real numbers in: actual weight, actual measured tyre width - and take the result as a starting point, not gospel.

Then adjust by feel, in steps. Drop five psi at a time over several rides. You are looking for the point where the ride is noticeably more comfortable and the tyre still feels supportive in corners. Go too far and you will feel the tyre squirming and rolling under you in a bend, and you will get pinch flats on impacts. Come back up a little from there.

Buy a gauge. Floor pump gauges are frequently inaccurate, and two pumps will disagree by a surprising margin. If you are chasing a pressure, you need to be able to measure it.

Measure the actual tyre width on your rims rather than trusting the label. A tyre marked 28 can sit at 30 or more on a wide modern rim, and that changes the correct pressure meaningfully.

And check before every ride or two - tyres lose pressure steadily, and the thinner the tube the faster.

The sidewall number is a maximum, not a recommendation. It is the pressure the tyre is rated to hold safely. Running it is like driving at the top speed on a speedometer because it is printed there.

What actually sets your pressure is three things:

Total weight on the tyre. You, the bike, and anything you are carrying. Heavier means more pressure. This is the dominant factor and it is why one number cannot be right for everybody.

Tyre width. A wider tyre holds the same weight at much lower pressure because there is more air in it. This is most of why pressures have fallen so much - tyres have got wider.

Weight distribution. More weight sits on the rear than the front, so the rear wants more pressure. Usually a small but real difference.

And the reason lower is usually better, up to a point: a very hard tyre does not roll faster on real roads. On smooth laboratory surfaces it does; on actual tarmac, an over-inflated tyre bounces rather than deforming over the surface, and that costs both speed and comfort. There is a pressure below which losses rise again, and most people are nowhere near it - they are far above the point where it stopped helping.

So the harshness you feel is not the price of speed. It is just harshness.

Worth knowing what changes the answer, since a single number for you is not stable either.

Surface. Rougher road, lower pressure. A route with poor tarmac or gravel sections wants noticeably less than a smooth one.

Weather. Wet roads generally want a little less for grip.

Load. Panniers and a laptop change the total substantially. If you commute loaded and ride unloaded at weekends, those are genuinely different pressures.

Tubeless versus tubes. Tubeless can go lower safely, because you are not going to pinch a tube against the rim. This is most of why tubeless riders quote such low numbers, and it does not transfer to a tubed setup.

Temperature. Pressure rises as tyres warm. Minor, and it exists.

On your specific situation: if you are on 100 psi and getting bounced around, you have room to come down a long way and the ride will improve immediately. Do it gradually so you learn where the limit is rather than overshooting and deciding low pressure is bad.

And if you are on narrow tyres on an older frame, the bigger upgrade is often fitting the widest tyre that fits. More comfort, more grip, no meaningful speed cost on real roads, and it lets you use the lower pressures the rest of us are talking about.