So I’ve got a gpx of a ride I did last year with a mate – but where I can’t remember the exact route. I’m planning to upload it to my new Garmin as a course and give it a go this weekend probably.

I’ve used the navigation on the road where it was amazingly good – basically like car satnav with a beep to warn when the next turn is coming up and a line on a map to follow.

Is it likely to work as well off road or is the mapping not great (given I’ll barely touch a road)?

No, turn-by-turn is unlikely to work well. Turn-by-turn is dependent on the underlying mapping to determine what constitutes a junction, and therefore a turn, and unlike roads where there’s a clear classification of “through road” and “side road”, this doesn’t really exist for trails.

That said, off road navigation does work very well. Just keep half an eye on what’s coming up, and it’ll beep loudly as soon as you’re a few metres off course. If you’re starting from a GPX of a ride that you actually did then that’s even better, as you don’t have to deal with the problem of trails not being where the map says they should be.

This sounds good then. Thinking about it I borrowed an ancient Garmin gps in maybe 2015 when I was on holiday in Spain. It didn’t have any mapping and turn by turn was just a bit wrong on that route. Hopefully with the mapping and the beep to say you’re off course should be much better!

Generally you’ll want to load a TCX not a GPX file to get turn by turn instructions but as @pdw says, off-road it’s best not to rely on it so don’t turn your brain off.

Generally turn-by-turn works by routing between junctions, fairly obvious on-road but off-road where legal ROWs and what’s on the ground differ plus you might have paths following firebreaks, animal tracks etc. you just have to keep your wits about you.

Cool – all I did with my road course was plan it on Garmin Connect desktop then save it as a course. When I clicked go on the unit it had turn by turn plus a beep plus the map. Even warned me about sharp turns coming up. No hills were meaningful enough over the 72kms for the hill warning function to come into use though. Will have to try that out at some point with a hillier route.