You need a powermeter and a chest heart rate monitor (capable of HRV readings) or a footpod and a heartrate monitor (for running), then after a month or so of collecting data it will give you independent numbers for cycling and running.

I'm 25 years old, 5'7" 135lbs.

For cycling mine currently shows 68 VO2max, I use a Garmin 820 Edge, a Wahoo Tickr X, and a PowerTap G3 power meter. I weigh 135lbs and have a 290 FTP.

For running (which I just started getting into...) it shows 55 VO2max, I use a Garmin Forerunner 735xt, a Wahoo Tickr X, and a Garmin Footpod (calibrated using GPS runs outside). I can run a mile in around 6:30 minutes/seconds.

Throughout the season the cycling number varies, going as low as 62 VO2 max during high humidity 90+ degree weather (makes HRV dip and % maximum HR go up for a given sub-maximal exercise, because that's how heat affects ya) and as high as 74 VO2 max during winter (when training inside at 60 degree temp with low humidity, perfect conditions). I'd be interested to see of the Garmin 530/830/1039 and their "temperature acclimation" update would cause this variance to change...