I plotted a route on garmin connect which should of been 152 miles I also double checked it on strava route creator and it was also 152 miles. Even google maps suggests similar.
When actually following the route yesterday the distance to go wasn’t decreasing proportionally to the mileage I was doing. At one point I had done 102 miles and still had 90 to go according to the garmin. I rode along watching the distance to go fluctuate up and down from 93.3 to 93.4 miles and this went on for well over a mile. I followed the route exactly and had backup strava on my phone and two friends phones and garmins said similar to mine.

I’m used to planning routes and having an extra mile or two but this was potentially getting on for an extra 50 miles. We changed route in the end and finished up at around midnight with 153 miles and 12000ft of climbing.

Any ideas?

If the maps on your Garmin don’t match the maps you planned the route on (for instance, a cycle route or junction has changed) then it could have been routing you a different way.

Is it possible that two satnav systems were interfering? My car Garmin says this can happen within one meter.

I cant work it out. I would expect a little bit of difference but not that much. Not everyone had multiple devices running and looking afterwards all our total distances were very close. I’m going to plot the actual route we took on garmin connect later and see how that compares.