I have a 2012 F350 I keep the hubs in auto all the time. In the winter I just flip the switch to 4 high when I hit snow conditions. I know to shift in or out I need to be below 55mph. How fast can I safely drive in 4 high on the interstate? I drive hundreds of miles at a time in the winter. Some times if the roads clear I will cruise at 70-80mph. This is a question of mechanical safety on truck components not driving safety.

 

You know that is a good question. I don't think it matters if the hubs are in the locked position as long as the Tcase isn't engaged.

 

If you can drive 70-80 mph, there is no need for 4wd. Actually if you can maintain more than 40 you shouldn't need it. The faster you are going, the easier it will be to cause damage on dry pavement that can cause driveline bind.

Ford doesn't seem to actually specify any speeds for the Superduty anymore.

 

i remember reading the Raptor can go up to its top speed 100 mph in 4x4

 

I have gone down the track a few times in my Super Duties. 104 mph in 4 hi with my 6.4.

 

some people worry too much, you can turn on 4 high at any speed pretty much. if I am driving 70 there isn't a need for 4hi, but sometimes I have forgotten to shift out of it and went those speeds with no ill effects. what I do, do however is I lock my hubs manually when I know the roads are bad and I will likely be shifting in and out of 4hi quite a bit. this makes for a smoother 4wd shift.

 

I would imagine that the driveline components are fine at any speed. My concern, as Matt suggested, would be driveline bind on drive pavement which can happen really fast if traveling at 70mph.

Another concern when driving in slippery conditions is making sure your front tires are free to rotate at your current speed which yields the maximum traction for steering. When in 4wd the front wheels are locked to your rear wheels which can affect their ability to rotate with the ground and lose traction.

 

I forgot about the track. I ran my 6.0 plenty enough times in 4wd, but I always kicked it out to 2H once it hit second gear. Some people would argue that it won't disengage under a load, but I saw a guy nearly hit the wall at Rockingham when he disengaged prior to the 2nd gear shift.

I still say high speed on the street isn't a good idea because of binding that could be caused by steering on dry pavement.

 

2 things. You shouldn't shift in/out of 4x4 over 60mph.

As long as you are on a low friction surface, you should not cause any damage travelling at any speed that the truck can do in 4x4 high.

 

Keep it under 30. No reason for any speed greater than that. Run it often though, meaning throw it in 4 once a month for about 5-10 miles just to make sure all the components are lubricating themselves etc.

 

When the road is snow covered and I don't have any weight in the bed, I will put it in 4x4. Otherwise the truck feels very squirrelly. I will regularly go over 40mph in 4x4.

 

I do the same thing. Have had to hit 4x4 a few times in this recent snow, pulling people out of ditches...

 

This is my opinion based on my experiences, it's more likely for the rear axle of a unloaded pickup to 'kick out' and fishtail when in 4x2 than 4x4... for that reason I keep it in 4Hi pretty much all the time except during low speed, tight turn maneuvers such as parking lots.
No road that is posted at highway speeds at is going to have any corners sharp enough to run into serious driveline bind. If it was really that sharp, then people would be rolling their vehicles around that corner in any weather or road conditions, good or bad.
It is also easier to recover from a fishtail scenario with 4x4 already engaged, instead of finally putting it in 4x when you are off in the ditch and your hubs are buried and you can't reach them to turn them.

I've owned a few 2wd-only trucks to know it's not enjoyable at all in the winter time... 4x4 doesn't make you invincible, but in the worst conditions, it can mean the difference between going somewhere very slowly, or giving up because you're just spinning tires when the roads have turned into curling lanes and you got no weight in the pickup bed to hold it down.

For months during the winter time, roads aren't going to be cleared down to pavement at all. They're just bladed to a level hard snow pack. My front axle hubs stay locked in the whole season.

But as the OP said... mechanical safety not driving safety... I think mechanically it is safe up to the vehicle's maximum speed... driving safety is a different thing and is often much lower. We're mixing up a discussion of what you CAN do with what you SHOULD do. The transfer case isn't going to explode because you went 1km/h or 1mph over.

 

4 hi on the track is the way to go...much better launch.