You asked the question is it within 1 mph at 25 mph? That is a 4 percent error tolerance, you are likely within 4 percent for speed which many would find good enough.

Many bike computers round off to the nearest half mph. Some round up to the nearest half mph, thus if you are going 14.1 mph some computers would read 14.5. So, computers already have a lot of error built into the readings.

But most people would want better precision for distance traveled. If you rode 26 miles and it said you only rode 25, would that be good enough? You decide. If you only look at it for speed and not distance, then distance error does not matter to you.

I use a tape measure and measure starting with valve at lowest point, as described above. There are 25.4mm per inch.

I like to keep track of my longer rides to the nearest tenth of a mile. Thus, I want more precision. But, I am an engineer, and engineers like precision.