Different size tyres : speedo error

[Fazda]

I used to check my speedometer calibration on an open road where you can drive at close enough to 120 km/h as possible (according to your speedo) and then measure the time it takes to travel 10 km, using the milestones (kilostones ?). At exactly 120km/h the time taken to travel 10km should be 5 minutes, 0 seconds. Your actual speed is calculated by s=d/t ie. 10km/time.

The only problem here is you need an open road/freeway with very little traffic, where you can maintain driving at 120km/h. Is that still possible?

That is certainly one way of doing it. I have found however, that the 200 metre boards on the freeway are great for doing it over a one kilometre stretch. The chances of keeping a steady speed for 1 km are far greater than doing it for a much longer distance. One kilometre at 120 is exactly 30 seconds. If you have a watch with a tachymeter all the better!

 

[Other Pineapple Smurf]

Nice post Fazda.

I can say from my mountain biking that I have to set my computer to the size of my tyre each time I switch. It makes a massive difference to the distance travelled and speed being reported.

Using the tyre size is only a guideline. The proper way is to regualary measure the tyre diameter by making a mark and then pushing the tyre a full revolution. You then measure this distance travelled.