Mine is always in 4WDA unless I ever need 4WD.

 

 

IMO if i had 4A which i don't, i'd only use it only when i needed it.
Like on patchy snow/icy, muddy, gravel, sandy roads, when raining or mild off roading.

I see no dire reason to use 4A for normal every day driving on bone dry roads.
2 wheel drive only and part time 4X4 trucks (when in 2 wheel drive) do just fine on dry pavement.

I'd think using 4A full time would increase wear and tear on the transfer case/ drive train and probably even result in a little poorer gas mileage depending on how, when or if the PCM decides to engage the front wheels.
My understanding when in 4A on dry pavement its in rear wheel drive until the PCM detects rear wheel slippage and then engages the front wheels for more traction.

 

 

To answer the question. No limit, but just because you can doesn't mean you should. If you think you need 4WD then you just might need to slow down and figure out why you would need 4WD.

 

How fast will your truck go?

 

As fast as you want - 70mph no issue

 

Its important to note that 4A on our trucks (I have it on mine) is NOT like a typical all wheel drive vehicle.
The 4A on our trucks will only engage if it senses wheel slippage from the rear. Unlike an all wheel drive vehicle that would pull you out of a slide, the 4A on our trucks won't do that.

 

I tried looking it up and only found that highway speeds (65÷) are fine in 4auto. Your front hubs will be locked so axles and front driveshaft will be spinning and transfer case will send power to the front when the truck needs it