Basic Gear Speeds

Most of the vehicles that I have owned have had tachometers.
My D-90 does not.
I am having a hard time judging by the engine noise what is a comfortable speed to run at in the gears.
I'm not talking about accelerating through the gears.
I'm talking about if you are cruising along in 4th gear at about what speed would you run before shifting into 5th? Does that make sense?
I am running 265/75/16 tyres.

Also, what about a tachometer, do any of you have one?
If so what do you recomend?

Thanks.

how long is a length of string

listen to the engine, if it labours change - if it's over revving change

least with a depender you get to hear the engine even with the windows up

Lordy...havent heard a rev counter called that for years...

I have 2 rules for changing gear...

1. When i cant go any faster...change up.

2. When i start to lose speed...change down.

rule, is the engine revving and working harder than it would if you changed up,
change down when neccercery

How fast would you run in 4th gear without changing to 5th?

depends on lots of things, uphill, downhill, towing, no good setting a speed to change up, work by feel, and noise