Great to see I'm not the only crazy one with ODB in mind.

Note that (on purpose) speed won't be reliable.

In several jurisdiction, it's illegal to under-estimate the speed.
If you go 81km/h but your car says 80km/h, the garage that did the setting will definitely get into trouble.
Thus it's common to put a (usually -5 km/h) bias in the reported speed to be on the safe side.

I would strongly suspect that the ODB reports the "corrected safe" speed.
(Haven't been hacking ODB, so I can't confirm. But any other car instrument such as adaptive cruise control seem to rely on the same speed).

So it might be useful to either give an opposite correction, or have the smartphone compute the correction while GPS is up.

Technically some of the embed satnavs *do* use the speedometer and compass as a fall back (random example, my parent's Volvo).
And on some cars, the satnav is an actually Android app running on Android Car (in my experience: on some Renault Zoé I've rented).
So there are other apps that do it, but it's a very specific exception (the app is running on a special platform that provide this), definitely not something that smartphone can provide.

(Well, maybe the comma.ai's openpilot would, in theory)

Some of the integrated satnav tend to try to guess from the car's compass heading.

In the specific case of Sailfish Apps, on platforms where it is supported, it could be possible to use the smartphone compass, provided that :
- it got calibrated during the time when GPS was available (i.e.: app sees that GPS reports the car driving straight ahead, but smartphone's compass is a bit "off" because apparently the driver has turned the smartphone toward them to have better visibility).
- the driver (or passenger) hasn't fumbled with the smartphone's stand while in the tunnel.

Shouldn't the turn lever be hooked on cars with Lane Departure Alarm System (LDAS), so it is communicated to the camera and the camera knows if a (dashed)-line crossing is expected or not ?
(If you change lane and put the turn signal, the LDAS is silent.
If you forgot to put the signal, or are slowly veering out of your lane, the LDAS sounds).

On non-hooked LDAS (such as some aftermarket dashcam, or satnav/dashcam combo like Garmin), the alarm only sounds on non-dashed (non-crossable) line crossing.
(LDAS never sounds on lanes, only when you reach the highway's borders).

Obviously, you haven't lived in a city with an underground and/or tunnel-bridge highway network. Great for reducing noise and traffic in the city, but can get a bit confusing when you need to take complex/multiple exits.
(I am currently living in Basel, CH - had something much simpler but still in Neuchâtel)

Luckily, in CH it's *clearly* indicated.
- satnavs that try to simulate the indication image (Tomtom) usually guess it right.
- satnavs that have a real-world picture of the actual indication use those successfully (Garmin).


I also think it's a great feature (I've seen it on the Garmin I installed in my mother's car):
"Speed limit change to {new speed} in {countdown} meters" showing up in the top warning box.