Tyre sizing.
 

Is it possible that the writte size numbers that are on tyre not correct?Meaning that if it says 23mm tyre,when i measure it,it shows 25,that's just anan example.
And can i measure the width of my tyres with a measure tape?

It often happens that bike tires are wider than the numbers on the tire. Part of the reason is that rims have been getting wider. My rims are 23mm wide and a 28mm tire actually measures 29.5mm. You don't need a an expensive machinists caliper. Hardware stores inexpensive ones that are fine for bike tires.

And it can go the other way too. I just mounted some 29x2.35 Schwalbe Big Apples that measure 2.15".

Also, road bike tires sometimes want to appear as though a given size is lighter so a 700x25 might measure 23.

The written sizes are not true. A more true and useful measure of the size is the ISO size formerly the ETRTO size. It's probably somewhere on your tire. Many times it's going to be in the tiny print on the sidewall of your tire. Sometimes they bastardize it on 700c tires and will put 700 x nnC or 700C x nn instead of the nn x 622 which is a proper ISO format for a 700c tire.

https://www.sheldonbrown.com/tire-sizing.html


I don't believe that the measurement is external either. I always thought it was the casing they measured to. So tread and rubber on the outside doesn't count.

You don't. The width is written on the tire's sidewall. They are always a little wider once inflated / installed. 25mm Giant tires are actually close to 28mm.

Thanks for help guys.