Ged has a Cat eye bike computer, which is giving him all the wrong readings, and he wants to know what to do about it. He has a couple of options that he can take, so we go through them here.
I have a Cat eye Strada computer that reads 3% too high. The distance has been checked on two maps and by a ciclspeed computer.
The wheel size is actually set to less than it should be so the situation is even worse.
I know that front lights can interfere but in the morning it is not on so it isn’t that.
Yes, you are correct. Front light can sometimes make the Speedo miss read, but it sounds like you have discounted that one.
Have you measured the wheel accurately? With the tire at the correct pressure roll the wheel out one revolution, the valve being the best place to start. The chart that comes with the computer can be a little out with its calculations.
If the measurement is right, then you need to set the wheel as slightly larger to compensate for the missing reading. Try to ride with a friend who has an accurate bike computer and compares speeds. You need to adjust your wheel size setting until you are both at the same rate.
Throw the old Speedo away and spend some money on a new bike computer that works properly, there are lots to choose from; you could go for one with a pulse monitor so you can improve your fitness at the same time. You could also get a Speedo that works from GPS. This is the most modern and effective way to record speed, distance, and all the other essential details you need.