# How bike computer wheel circumference change affects speed

I have a cateye stealth evo+ bike computer and have been using the standard 700/25 wheel circumference that is listed in the instruction manual "2096". The value has been changed to 2143 using the rotating wheel method(stem to stem) while on the bicycle. The wheelset and tires are the same. Will this increase or decrease my actual and average speed.

## 2 Answers

Your actual speed will remain the same. The speed displayed by the computer will be higher by 2%, as Will Vousden says. Please note that this higher reported speed will probably be more accurate (closer to your actual speed) because you measured the true circumference of your wheel (which is what I assume you mean by the "rotating wheel method").

The cycle computer will over-report your speed by a factor of `2143 / 2096`, which amounts to about 2%. The speed calculated by the computer is `circumference * cadence`, so if the circumference is 2% too big, then the speed will also be over-reported by 2%.

Conversely, your actual speed will be about 2% smaller than what your cycle computer reports: a factor of `1 - 2096 / 2143`. So if the computer says you're moving at 30 km/h, you'll actually be moving at about 29.3 km/h. The average speed will scale in exactly the same way as the instantaneous speed.

In general, if your true wheel circumference is `a`, the value you've put into the computer is `b`, and the reported speed is `v`, then the true speed will be `a / b * v`.