I have a wired computer and bought a wireless as a backup.
Set wireless to the same diameter ( 2168 700x35 ) as the wired and logging less mileage ( 1 mile) on wireless than wired.
I usually ride the same route so I know the wired mileage is right,
Should I change the wireless diameter up or down to come close to the wired?

If you are going to use two devices, then use the one you think is correct and ignore the other. But if you want, you can fudge the wheel calibration.

Both devices use the same wheel to get a distance?

if you have a sensor that reads a rotating magnet attached to a spoke, I would double check that as to alignment and position. You may be missing some wheel rotations.

both computers us 2168 for 700/35. gonna try going up on the wheel calibration till i come close.
Had a computer "die" out in the middle of nowhere that's why the backup

Have you been vaccinated with the Pfizer vaccine? That can interfere with with the wireless communication and take some of your miles away. Happened to a friend of mine.

got Pfizer, removed the chip prior to installing

What about Moderna chips? <Insert chuckle.>

My wired computers (Sports Instruments, company went belly up years ago) works spot on, every wheel revolution counted and no extra revolutions. But I have some wireless computers (VDO), they appear to send a packet of information to the computer every few seconds instead, but the wireless ones are often wrong by a few percent for distance. I mostly rely on the wireless ones for cadence, moving time and speed (they round off to nearest half mph which is within their error), but I do not rely on my wireless ones for ride distance.

What is the total distance of the ride?

These work by counting wheel rotations and multiplying the number of rotations by the circumference.

If the circumference for both is set to the same number and they are counting wheel rotations properly, they should produce the same distance.

If you are using the same circumference and getting different numbers, then one or the other (or both) is counting rotations incorrectly.

The usual problem is magnet alignment,

Wireless can also have interference.

Digital wireless should lose rotations (yielding a number that is lower than actual) if there's a problem.

Non-digital wireless might measure spurious extra rotations.

Unless the number of dropped rotations is close to constant, changing the circumference won't help much (that isn't fixing the problem).

When that happened to me, I junked the wireless and bought a wired 'puter. Later I bought a GPS-based 'puter and a speed sensor, because my goals - besides enjoying the rides - are distance-based, and I didn't like losing distance.

Thanks for the info passed on. I will check out a few things we=hen it gets a little cooler this week.
I also will be getting RidewithGPS soon so I will probably use that for a backup