im sick at the moment, and thinking hurts when im healthy so i put it to you NR: i have a slight concern regarding the possible variation in my speed. fair chance this has something to do with an avid hate towards speed cameras, and my ability to attract their attention. i normally run a 140/70 tyre on the rear. the bike i just purchased came with a 140/80. my understanding is the second value is a percentage of the first - the tyres i just got have a 10% larger wal height in relation to the tyre width. thus the radius of my rear is increased by 14mm, and will increase my actual speed compared with my percieved speed. what i want to work out is how much the speed increases. understanding that C = 2Î R, an increase in R increases C by a factor of 2Î . so irrelevant of the original Circumference, the new circumference has increased by 14mmx2xÎ , or roughly 88mm. but to work out an increase in speed, we need to know the original circumference. im taking a punt assuming i can say with a 17 inch diameter rim (215.9mm radius), and an original tyre wall height of 70% of 140mm, which is 98mm, my old tyre had a total radius of 313.9mm. this results in a circumference of approximately 1971.292mm. thus the increase in circumference is almost 5% (4.4% i think), and as such any speed i read on my speedo, i should add 5% to be safe. this doesnt take into account any inaccuracy of my speedo already... does this sound right mathematically so far to you guys?