I can recover ok when I'm in high range. But when in situations when the truck is suppose to be in low range that's where I had trouble. I think I was rev-ing up too much.
You know, this actually does sorta work. It's not exact but it's a close enough approximation. Yesterday, I paid attention at what speed I was doing during my down shifts and by and large, I was a tad faster than what Pumpkin Oval Head mentioned. In my 10 speed, I shifted down 9th - 53 mph 8th - 40 mph 7th - 35 mph I also found if you also, take your speed divide by 5 and that will get you a close approximation to the gear. Another bit of advice to the OP is practice downshifting in the upper range to get your technique and rhythm. It is more forgiving than lower range.
Actually you'll need to practice a little bad math and go one gear higher or you'll just grind the gears (example 15mph 1+5= 7th gear).
The math thing is funny. A 18 a 13 and a 10 speed all need 6 or 7th at 15mph? Know your truck or be ready to put it in the ditch, thats it. I find the biggist thing to know in a sudden slow down situation is knowing your hi-low break point, i.e. 25mph or so. Not hitting that spliter unless you need to.
I've only driven Freighliners and Internationals but your problem is more than likely just not reving the RPM's high enough. You'll usually need to go much higher than you might think to get the low range gears.
I suggest to take a few off ramps and practice down shifting. Use your brake of course to slow her down and throttle up if needed. Barely push the cluth in.. just enough to change a gear.. I 'm in a 10 speed myself at the moment just different breed. It'll let me down shift or up.shift on a roll with little to no throttle. go thru the gears from a stop and check the speed and rpm that works then each truck is a different animal