So I have started to ride with a group now (which is way more fun than alone), and at the end of our long rides we all compare computer information, average speed, etc.

What I have noticed is that most of the people I ride with all have near the same results in distance and speed and all. My computer seems to be off... and I mean by like a half mile.

So I started to think the programing was off, tire size in the computer wrong.

So I looked on my bike to double check tire size and came up with some interesting discrepancies.

My rims say 700x25c
however my tires say 700x23c

I am not an expert on this, but my computer is programed based off my tires the 23c, would that make a difference that much? should I keep the 23c setting or change it to a 25c?