Randy, I think you can't go wrong with 70mph, but I'm sure that working with your plane to be exact can't hurt either. The chart that cub flier posted will have all the important variable. Best glide (most distance over time) is generally close to L/D Max. If that takes anyone back to some confusing aerodynamic lessons. It's simply the most lift from the wing possible for the drag spent. That drag, in turn, can be increased by speeding up (more parasitic drag), or slowing down (more induced, or lift-created, drag).
The 70 MPH sure is a number for plain factory, gross weight, supercub. Big tires might affect parasitic enough to slow best glide a couple mph. A really lightly loaded plane might also move best glide lower, by reducing lift req(induced drag). Best glide should more be a pitch attitude that speed, but it's probably simple with speed.
If your keeping 70mph all the while running emer proceedures, selecting an evaluating a landing spot, shutting off fuel, mags, popping a door, pressing 911 on your spot, squawking 7700 and talking on 121.5, you're better than most.
Lastly if you think of it, or have time to consider..... Glider people think of Optimal glide and add 1/2 estimated knead wind to Vbg. Or subtract 1/2 estimated tailwind to Vbg. Also consider, if over water, or over an airport, A Vmd or minimum descent (or least sink) might be your best option to give you the greatest amount of time for a restart or getting things together. I hope you don't need any of it. I'm not a gider guy yet, but, I've been thinking about it and researching their material.
Of all the single engine planes I've flown, the cub is my number 1 choice to lose one in, and not for it's great glide.:smile:
Cheers.