On the Canadair Regional Jet that I fly, the L/D best glide speed is
different at all altitudes. We have a chart that shows us what
"indicated" airspeed to pitch for if both engines shut down for each
altitude. As altitude increases, the best L/D speed also increases.
For instance, at 10,000 feet the best glide speed is around 170 knots
indicated. Then at 35,000 feet the best glide speed is around 235
knots indicated. This speed is if both engines shut down, and then
cannot be restarted.

Also, at 35,000 feet our indicated airspeed is around 260 knots and
our true airspeed is 450 knots at mach 0.74 when in level cruise
flight.

Hope that helps. Most of my soaring is in Arkansas below 6,000 feet
msl.