Trying to figure out which tires/tubes I can use

Question

I have a road bike tire with the following markings:
50-559

26x1.95

K935-001

I weigh about 200lbs and will probably carry a pannier holding 40lbs. Some research said that my tire should inflate to about 110 PSI. This tire reads that it doesn't support a PSI over 65 (bought the bike used). So I figure I need a new tire.
However, from here it’s been hard to research what I should get.  When I search Amazon I don’t find a lot of tubes that describe the width as 28mm.  Instead I see a lot of measurements like 26x1.95, which is the dimension my current bike has.  I figured maybe the 1.95 represent the width in inches, but when I convert it to millimeters it’s about 44 millimeters, way bigger than what I found recommended.
So I’m just not clear one how to reconcile all these incomplete and contradictory-seeming sources of info.  All I really want to know is:  When I go on Amazon to get the tubes for my bike, which measurement (or ranges) do I need to search for to be sure it fits and won’t explode under me?

Criggie · Accepted Answer

Some clarifications

A tube does not have an air pressure maximum - it is restrained by the tyre and rim (*1)
A tyre does have an air pressure maximum
A wheel/rim has a weight maximum but not an air pressure maximum (*2)

So your combined system weight is going to be about ~260 pounds (120 kg) by the time you include the bike, the panniers, the rider, and contents of your pockets/bottle.
Most conventional bikes have about a 40:60 split, so 40% of your weight is on the front and the rest is on the rear.  This changes when braking or accelerating, or if you've bounced through a pothole.  Upshot, your front wheel is holding about 104 pounds (48kg) and the rear is about 156 pounds (71 kg).
At 65 PSI in each tube, your front wheel will have a contact patch of 1 3/5 square inches, and the rear will be 2 2/5 square inches.  This is the area of flattened rubber that touches the road to equalise the total mass.
Your 50mm (2 inch) tyre will compress by some amount to make that surface area.  So the energy to deform that tyre initially comes from the weight, but to roll forward the energy comes from your pedalling and that's why big soft tyres feel sluggish.
To put that another way, a higher tyre pressure deforms less so saps less energy to roll.  (*3)

The max and min width of tyre your rim can take depends on the inner rim width.  There's a full Q&A on that subject here:   What is the maximum or minimum tire width I can fit on my bicycle
Bike tyre measurements are not simple - there are a crazy number of systems where a bigger number can be smaller than a smaller number.
For your existing 50mm 26" tyres, you need a 50-559 (to use the ETRTO system)  which is the same as a 50mm wide for a modern 26" rim.  Just make sure you have a third tube as a spare.

Finally - I don't know where the 28mm figure comes from.  Its possible you've seen something about modern road bike tyres whose widths have increased from 19mm to 23 to 25 to 28 on pro bikes, but these are for a different rim size and style of bike.
Remember, Amazon is not a bike shop.
They're just a box-mover who wants to sell stuff.  If you need practical real useful advise, a Local Bike Shop/Store can be much more helpful.  Or ask here on Bicycles.SE, or ask in [chat].

yes a tube will eventually pop if inflated outside a tyre, that's not what I meant
yes a rim/wheel will fail if the air pressure is too high AND the rim is worn
to a point - there's a sweet spot in pressure, where too much has no grip.  We want the goldilocks pressure for each wheel and conditions.

BetterSense · Answer

50-559 means 50mm wide tire, for a rim that measures "ERTO 559mm" in diameter. This is a very common size.
The "inch" equivalent is a tire for a 26" wheel, with a width of about 2 inches. Any "26 inch" tire that's about 2 inches (or 50mm) wide will fit your bike, such a 26x1.95. The difference between 1.85, 1.95 and 2 widths is not large.
You could potentially get a wider tire, like 60mm / 26x2.4 inch, and it will fit your rim, but you would have to check if the wider tire will rub on your frame or fenders. Wider tires will ride better, but be heavier.
You could also get a narrower tire like 35mm / 26x1.5, and if should fit too, but the current trend is towards wider tires. Narrow tires will ride worse but be lighter.
Tire pressure that you should use is relative and will depend on your weight and the tire size. The number printed on the tire is the maximum tire pressure not to exceed. Usually, you will run less pressure than the max. On my cargo bike I run 26x2.5 tires with a max of 60psi. I actually run 35-40psi.
Tubes will work for a range of tires and you don't need to be exact. Practically any 26 inch tube between 1.5 and 2.5 inches will work for that whole range of tire widths. So you can use a 26x1.95 tube for a 26x2.25 tire, for example. The important thing is the correct wheel size (26") and a width that is reasonably close to rhe tire width.

Trying to figure out which tires/tubes I can use

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Remember, Amazon is not a bike shop.

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