I recently bought a Cube with the Intuvia display. The first thing I noticed was that the distance recorded was much higher than my Garmin GPS. I reduced the circumference setting over a few rides until both were reading the same.
you only have a tiny bit of range you can change. but i have seen that bosch tends to be on the faster then actual size. so make the wheel size smaller and yoru fine. hell I dobut you can change it much more then a mile or two per hour.
This is how the calculator assesses the wheel circumference of my 47-622 Schwalbe Smart Sam tyres...
The point is, the exact profile of the tyre, and the exact height of knobs makes the wheel circumference different from the estimated value. I have found the actual wheel circumference of my Vado tyres at 4 bar inflation was 2216 mm (and that value has given a 100% match with the GPS on multiple long rides).
The same for my 37-622 Smart Sams on my Vado SL (2157 mm). The size of 37 mm is even not in the calculator!
If I had a tape rule longer than 2 m, I would have certainly tried that Precision within 1 mm is hard to be attainable though Why not simply calibrate the WhC value against GPS?
Richard: 10 mm error gives half a kilometre discrepancy over 100 km. That's why Garmin Edge computers involve Auto Wheel Circumference capability to calibrate the parameter.
You owners of Intuvia and more advanced Bosch displays are lucky people, as you can set the WhC value yourselves. Not doable for Specialized (the dealer can do it for you).
I understand that. Every serious e-bike brand takes care that users do not tamper with the wheel size to go beyond the speed restriction more than the allowed +10%.
An anecdote from my own experience:
My Vado 5.0 came with 51-622 tyres, which I replaced with 47-622 after a couple of months. Of course the e-bike display went crazy because of different WhC, so I have been relying on the GPS for speed and distance ridden. Near to the warranty expiration, I asked a Spec tech to set the WhC to 2216 mm for me. After he connected Turbo Studio to my Vado, he became silent and seemed to ponder over what he saw on his screen. Then he announced:
-- It is very very strange. The Wheel Circumference value seems to be locked there at 2300 mm. I cannot change it! -- to which I brightened up and explained it to him:
-- It is a moped. A type approved moped. The Certificate of Compliance reads the type approved tyres shall be Specialized Electrak 2.0. That's why you are not allowed to adjust the wheel circumference here!
-- Oh, I see. Proper tyre air volume and all...
(He could adjust the wheel size for my Vado SL, as the latter is just a bike in Europe. Still, there are some adjustment limits).