Discrepancies between Cyclemeter and Strava Distances

It should be pretty easy to check. Get on the turbo, top gear, pedal up to 100 rpm and see what speed. Assuming 50 on the front and 11 on the rear you should see around 35.8mph. For any other ratio 7.87 * front/rear will give you a good enough approximation of the speed. If that is correct, you need to make sure cadence is being counted correctly. Count rotations for a measured time to check that.
It could also be poorly placed magnets, the magnet on the crank may be too close to the speed sensor, or the magnet on the wheel might be too close to the cadence sensor. (Edit: Looking at the differences you are getting I would say this could well be the problem. Your cadence magnet could also be registering on your speed sensor, try adjusting the position of the magnet realtive to the sensor)

GPS, while often very good at giving you an x,y coordinate, is really bad at calculating altitude. If you upload a ride from the Strava app, Strava uses map data for the x,y point of your ride to get the altitude, completely ignoring the GPS. This generally results in way more accurate elevation for a given ride.

If you don't use the Strava app and use your own dedicated GPS, this is what the "correct elevavation" button in Strava does - replaces your GPS unit's altitude numbers with values from their map data. If you have a good quality GPS it will have an altimeter built in to calculate altitude instead of using the GPS value. This is what the Garmin Edge 800/810/1000 do for example.

2099 sounds about right - but what PSI and weight are you ? If its low - then surely that would reduce the figure and increase your milage ?

I'd put in a silly figure - half or double it to see if that is where the reading is being calculated.

Strava is accurate for me - it always compares well to my Garmin anyway.

So I'd go with that in future.

I use Cyclemeter and Strava with a Wahoo Blue SC and the readings are always pretty similar - never identical due to the issues described above but very close.

Even if you were at sea, it would depend on the state of the tide.

Speed/cadence sensors aren't particularly accurate devices; they're simple magnetic switches which can have errors from all sorts of reasons - wrong gap. too wet, battery expired (causing transmission errors to the head unit), etc. Strava will ignore the part of the .tcx file including the speed sensor info, and in this case will use the far more accurate GPS info. Not that GPS is infallible - and GPS altitude information from cheap receivers such as those on bikes is very poor.

The Bryton/Strava rollers session problem is a pain - I agree that if you've done an hour at FTP you should be able to analyse all the data, including speed and thus distance. I use a Garmin head, upload to Connect which then distributes to Strava. Alternatively, I use TrainerRoads (via an ANT+ USB dongle into the 'puter), download the session and then upload it to Strava. Either way seems to work ok.

I uploaded my Bryton onto Strava yesterday & my 36.7mile turbo session from yesterday morning turned into a 0.3km ride at 0.2km/h average.

from what I understand GPS doesn't do altitude very well, I remember being out sailing once and the GPS on the yacht said we were 3m below sea level - am pretty sure that was not correct.

You'd like to think so... But logic goes out of the window when anything to do with computers is involved

Have you allowed the real GPS position on your iDevice? If you don't then the route it plots will be based on cell tower info and in a rural area that can be miles wrong (positionally)

https://strava.zendesk.com/entries/22533685-Strava-iPhone-app-and-new-iO... (this for the settings you need to look at)

I cannot speak for Cyclemeter but when I have used the Strava App on my phone and also uploaded to Strava from the Garmin there has always been a big discrepency in altitude. I mean a BIG discrepancy like up to twice the amount on occasion. No idea why.