I use a Garmin which is pretty accurate provided that you don’t change your gait too significantly. A long time ago in college I used to have a very long stride. Many years later I took up running again and did some reading on it. Apparently shorter strides are better, I went out and tested it on the track and was more than a minute faster on a 5k.
The one problem with shorter strides and us slower runners is not going through the full range of motion in your legs. That led to an injury in my case. After taking off a few weeks I went back on the treadmill and while warming up exaggerated my gait so that my heels were almost making contact. There was a immediate sense of relief from the muscles that had been so tight before.
Running at 8 mph, the Garmin is quite consistent, varies usually no more than 0.2, but I noticed in the warm up when really kicking my legs back (same speed) it went up to 8.4/8.5. I can’t hold that gait for long (maybe 20 seconds) as it takes a lot of energy, it does seem though that the Pod gets confused by the larger range of motion.
Am looking at another treadmill that can directly interface with Zwift to get around the inaccuracies of pods.
I’m initially much faster (by 2 minutes on a 5k) on a treadmill than running outdoors, but once I get fit the difference goes away and it’s actually a little harder on the treadmill for me. I feel that outdoor running develops more muscle mass.