Tyre sizes

[TAV]

Hi I'm looking at changing my tyres on my gravel bike from 659b 47mm to 650b 42 so I can use it on the road. The problem I'm having as I've seen the tyres but they have 2 options 27.5 1.75 or 27.5 1.90 whats the difference.

 

[Cycleops]

One's a teeny bit wider than the other.

 

[TAV]

But how do I know which one to choose?

 

[Cycleops]

Got a coin?

 

[Jameshow]

1.75 is about 36mm

1.90 is about 40mm

 

[TAV]

Had a look it Sat's 42mm