No hubiésemos necesitado el GPS salvo cuando descendimos de ella

No hubiésemos necesitado el GPS salvo cuando descendimos de ella
 

La carretera que tomamos era bastante amplia y estaba bien señalizada. No hubiésemos necesitado el GPS salvo cuando descendimos de ella.

Both sentences relate factual things that happened and there is no doubt in anything.

So why is the subjunctive used in the second sentence "hubiésemos". "salvo cuando..." is the conditional part and the verb is indicative which is fine cause to my understanding, there is no doubt/uncertainty in anything.

Muchas gracias :)

I think it may be an Iberian way of saying apenas no necesitamos el GPS.
It looks odd to me as a user (but not native user) of Latin American Spanish. Native speakers will come up with something more definitive soon.

Subjunctive pointing to a non-performed action:

no hubiésemos necesitado ---> in fact we needed it

The sentence is a bit strange or maybe sloppy, but not because of the subjunctive. "Descender" is something you do from a highway, with accent in high, not from an on-level road, no matter how important, and wide, and highway-like it looks. I'd rather say "no hubiésemos necesitado GPS salvo después de haberla dejado". Las carreteras se toman, se dejan, se entra a ellas o se sale de ellas. A las autopistas se sube y se baja, se asciende a ellas y se desciende de ellas.

Thanks for your replies

More context might clarify. I've been in parts of Andalucía where you really do descend from the road into the ramblas, and that's where you're most likely to need GPS.