Tire size/Speedometer calculator?

Hey Folks! Just changed out my 2.75 x17 front tire for a 3 x 17 on my Honda CA. I realize that the original was a 2.5,so my speedo was off just a bit already, but I'm trying to find a fairly accurate calculator of how much it's actually off (no GPS available). Any links that anyone knows of, or, even an educated guess?

Those numbers designate width, not circumference. How much taller is the new tire as opposed to the old one. I doubt the .25" difference in the tire is enough to noticeably change the speedo. How did you determine the speedo was "off just a bit"?