I’ve had a cateye Vectra computer attached to my bike for a year now and i’m not sure of its accuracy as it doesn’t match other riders recorded distances..The tyre size settings don’t make sense as they vary depending on tyre width and there isn’t a setting for my tyres which are 26×2.25..see picture

Anyone any ideas what i should be setting it to??

best bet is to put the valve stem at the bottom of the rim and mark the ground where it is. Then sit on bike and roll forward until valve is back at bottom of rim again, mark the ground.

Then measure distance between the marks – this is the circumference which I think is what you are looking for.