The distance and strength don't really matter so long as it's enough to close the reed switch inside the sensor. If your bike was reading distance but not speed, it wasn't the fault of the speed sensor - it was the result of the error. You are already aware of the potential problem with the low gears which puts you outside the possible range of cadence and speed map, which is good, but what Shimano did was just constrict this range further and further until it became non-viable for any meaningful gain in speed (no errors with 1.2:1 ratio but that's a negligible bump in speed). Since SPEEDi is basically a copy of the old Shapeways LSS with some features mixed in from PLANET3 (such as mounting it on the bolt heads), it was a matter of time before this started to happen. I tested this at least 2 years before Speedi was even on the market and yes, everything works for a few hundred miles or kilometers (the ratio I tested was 1.4:1 as 1.675:1 resulted in errors rather quickly) but then two bikes started to receive limp mode errors. That's when I decided it was not worth playing with people's warranties and started replying to every Bosch inquiry that PLANET3 isn't supported because it is detectable by Bosch after some time. The amount of time depends on your use case: for people that did more technical climbs in low gears - they got the error sooner. For some who used the bike for commuting they could get away with 1500 km before the error appeared, but it did appear sooner or later. The only way to reliably derestrict a Bosch motor is via a chip, but even here you have a problem that Bosch themselves are cheating - what I mean is, they basically say in their algorithm for detection that if your bike is never ridden OVER the speed limit WITHOUT power - you have manipulated it. This is, of course, technically wrong and immoral of them, but hey.. it's the company behind Diesel-Gate. Imagine the following scenario: you load your bike onto your car, go to the trail head, ride it uphill below the speed limit (physics will set it so), then descent over the speed limit but without pedaling. Do 1500 km of this and you will be in limp mode error with a completely stock bike. They simply pronounced that never running into the speed limit is "impossible". This is why other chip manufacturers now advise to put the speed limit at 35 km/h or to some value where you still run into the speed limit from time to time. However, I've made a full derestriction device for Bosch but am still testing how to implement this occasional speed limit hit i.e. you'd have a power cut for 50 ms every few kilometers or so, telling the motor that the speed limit was touched but it wouldn't be something that you feel. That's how they handle the nominal vs. peak power problem. Bikes all have a nominal 250W sustained power output, but we know all of them basically have 700W motors that should be illegal. The workaround is that every 30 seconds or 1 minute depending on the manufacturer, there is a current drop in the motor that takes it from 700W to 250W for a couple of miliseconds - just enough to satisfy the "letter of the law" that it wasn't "sustained power over 250W for more than 30 seconds". Anyway, the world is full of unscrupulous people who are ready to copy and sell products without worrying about warranties and potential problems. Be careful out there.