Glide angle is determined by lift and drag. If the airplane is flown at its best lift/drag speed it will glide farther. Here is a quote from the F-51D Pilot's Manual (I flew them too) on a chart for power off gliding distances. "You will glide approximately 14.5 statute miles for each 5000 feet of altitude lost by holding 175 mph-no-wind condition." That is almost three miles of glide for each thousand feet of altitude lost or, a glide ratio of 15.312' for each 1' of altitude lost.
Nearly all jet airliners glide about three miles for each thousand feet of altitude lost. For example, it takes about a hundred miles to lose 33,000 feet at idle thrust and this is true for the B-707, B-727, B-747, DC-8, A-320 and L1011s that I flew, and this worked OK for a normal high speed descent. We always used this for a rough estimate for beginning descent. Idealy, we would stay at cruise altitude as long as possible where miles per pound of fuel burn were much greater than at low altitude. The ideal would be to fly the descent and approach at flight idle, spooling up only when configuring for landing. The Lear Jet doesn't manage quite 3 miles/1000, more like 2 3/4 miles/1000.
Now, instead of using "rules of thumb" for calculating performance, flight director computers do it. One simply tells the computer the "cleared to" low altitude fix and the altitude and speed needed to cross it. The computer tells the crew when to begin the descent. The flight director is then followed which commands the pitch attitude to give optimum Mach and Indicated Airspeeds to make the crossing as cleared with minimum burn.
5,280' (one mile) times 3 equals 15,840' (three miles) for each 1000' lost for a glide ratio of 15.840 to 1. And yes, this is a heck of a lot better than a C-172 because Jets and Mustangs are so much cleaner aerodynamically. Still, to put this into better perspective, Jet airliner gliding ratios are only 1/3 that of high performance sailplanes.
Al