I down loaded urban biker app this week I have made one ride with it . The cheap ( schwinn ) Wired computer on my bike showed 16.4 phone showed 16 miles I am guessing the gps will be closer to the Actually mileage ??
I usually ride 8-20 mile trips,
All I want is
trip distance
Average speed
Odometer
Altitude and max speed are interesting but not needed
I ride bike trails and known roads so mapping isn't a need
My concern is battery life , I am guessing that not running the app screen will increase this
Plus I have battery bank chargers
My thought is I can pick up a top tube phone bag just run my phone on all my bikes and have less devices. Also with different profiles the phone will cover all my bikes and have the same device measuring miles (consistent mileage across all bikes )

If my assumptions are wrong feel free to point me in the right direction and or point out a simpler or less power consuming app
Roy

How do you know the 16.4 miles is correct?

GPS may show a slightly lower distance than a correctly calibrated wheel rotation counter since the counter is including every wobble (you aren't riding in a straight line). You can also lose GPS reception (which will tend to lower the measured distance).

Counting wheel rotations is easy to do (wired is nice and simple). Being cheap isn't any problem. The issue is using an accurate wheel circumference.

GPS is still very good and good enough.

A lot of GPSs measure long, eg if you ride 100 miles they might tell you it was 102 miles. You're riding in a fairly straight line most of the time, but GPS has a lot of jitter. Plus, depending how you carry your phone (how much view of the sky does it have?) and how much tree cover, there's more noise. Anyway it's only 2.5 % difference, so I wouldn't sweat it all that much. (But maybe check the wheel size in your non-GPS bike computer.)

Otherwise all your assumptions are correct.

I took wheel size from instructions on the bike computer. I do know they are at least consistent as I have 2 of the same computer on bikes with 2 different wheel sizes and the read the same with in a few 1/100ths on both bikes . I ride the same trail a lot with both bikes. But I have 3 bikes without a computer. I am trying to simplify things
Thanks for taking time to help
Roy

That number is usually fairly close. But things like the actual tire and pressure might mean that's not quite the correct number.

It doesn't matter what computer you use. The wheel counter computers are (basically) all equally accurate.

If the computer is cheap, add one to each bike.

The computers are good enough. The GPS is good enough. It's not rocket surgery.