If you're avoiding the extremes (spinning in the lowest gear and grinding in the highest gear) then I think speed/mileage should be fairly accurate. My top gear is about 4.5 times faster than my bottom gear so it'd be really hard to use either at an overlapping "road" speed.
Heart rate may be the best to use, though.
Last edited by Herzlos; 03-09-21 at 08:01 AM.