Do any of the computers which utilize GPS and not a sensor show realtime Average MPH?

This is simple math and a feature they all should have. I will be completing my first century soon and the average MPH would be extremely helpful but only if it was displayed realtime and not something I would have to download from an app.

Thank You

I don't know of one that doesn't. Even some cheap wired computers can display average miles per hour. If the computer doesn't support user defined page setup to where you can place the data where you want it, you may have to change the display page to see it, but it's there.

Garmin 500 will show both average speed for the lap and for the entire ride.

GPS devices can't deliver realtime speed. There's always a slight lag, usually a few seconds. Whether this matters depends on the terrain. It's close enough for longer uninterrupted climbs, flats and downhills. It's not useful for the roller coaster hills in my area so I don't bother looking at my GPS devices for realtime speed while riding. However they can deliver near-realtime estimates of average speed. Depends on the device or smartphone app.

I use the ridewithgps app on my phone. The speed readout varied wildly so I increased the interval for averaging, which made the updates slow. I do read it and find it useful as long as I remember that it's not going to be very accurate and to the degree that it is accurate, it tells me my speed a few seconds ago, not my current speed. I think there is some recalculation after the ride's end, because overall average speed changes after I stop the ride.

I've been thinking of getting a speed sensor, but I'm not terribly motivated to go through the expense and trouble. It's not that important to me.