I have a mate who consistently posts quicker times than I, although we ride together + start and finish together. He uses the
Garmin speed sensor to record speed and distance, where I use the Garmin GPS to record speed and distance.
He has manually entered the wheel diameter. Would or could this make a difference? [minor typo edits]
Yes.
Pedant corner: If you and your mate "ride together + start and finish together" surely for that ride you are 'posting' identical times - how could his time possibly be 'quicker' than yours?
The Edge 800 and others, with a 'speed sensor' fitted, use the distance determined by received GPS signals to calculate the wheel circumference (by determining how many revolutions the wheel has rotated over a distance ?1000m). This calibration (given it's done over a km or less) cannot be accurate to closer than 0.5% (maths available on request). Once it is thus calibrated most Edges continually use the GPS data captured during rides to check and adjust the value (but again using a relatively short distance). This, for example, will make 'automatic' adjustments for a (slightly) different tyre (height) and a change in the tyre pressure used: we're talking millimetres in a wheel/tyre circumference of 2100mm (say) here: 8mm difference is less than 0.4%. In addition, those Garmins (and others) also let you specify the wheel circumference.
In the context of this thread, Garmin Edges (so enabled with speed sensor/Ant+) allows one to set the speed to the speed sensor calculation rather than the GPS one. Most riders default to GPS. But a regular turbo rider, too lazy to press a couple of buttons when starting a turbo ride, might choose to set the speed to the speed sensor calculation without any GPS involvement. I'm guessing the OP's mate has set their circumference too high. If they want to continue kidding themselves and it makes them happy, that's fine.
He should just turn the manual wheel setting off. Garmin do a very good job of automatically setting the wheel sizes via the GPS.
It's his mate's choice what to do.
If I return with a 16 mph average he will be a least at 17mph
I use GPS only, no speed sensor.
To be honest I have never checked the distance comparison with him.
Well next time you and your mate compare, compare the distance you've cycled together and then ask them why the discrepancy. Personally I would plot the route you've ridden on RidewithGPS and see who's 'closer' (answer: you will be - he will have a distance 6+% too high (100/16)