I would say no faster than 20mph or so... Why do you want to drive fast in 4wd? It also depends on whether you're in 4wd hi or 4wd lo. Low has a max speed of like 5-7mph I want to say. I also want to say that high is up to 15-20mph. Hope this helps. I'm sure someone else will chime in if I'm wrong.
In 4hi, you can safely go up to the speed limit, about 55 or so. I've driven my Suburban, '04 2500 with 8.1, up to 65 before on the interstate, I didn't realize it was still in Auto mode, but it was fine, even though it made a whine. Some diesel guys run the 1/4 in 4wd, and they get up to 85-100 mph. I wouldn't recommend sustained use of 4wd on the highway, but it can be done. The manuals say you can shift on the fly up to about 45 or so. And I've done that.
As far as 4low, I guess you can go as fast as your rpms will allow. I've gone about 27 mph in mine, taching about 3500-3700 I think, in OD. LOL Again, just testing it out, but I'd limit 4 low to about 10-15 mph.
There are no real mechanical limits (as far as the drivetrain is concerned) to how fast you can go in 4H. However, 4H should only be used when conditions allow the wheels to slip to prevent drivetrain windup and possible parts breakage. If conditions are good enough that you can safely drive 50,60, 70+ mph, then you probably shouldn't be in 4H. If conditions are bad enough to safely use 4H, then you should be very judicious in how fast you are going. Remember that 4wd doesn't do anything as far as stopping ability (all cars have 4w brakes), and does next to nothing as far as turning ability. In summary, the top speed in 4H is limited to a safe speed for the road conditions.
In 4L, your top speed will be limited by engine redline or whatever you feel cofortable running the engine up to. I frequently run up to ~30 mph in 4L when I don't want to have to constantly shift back and forth from 4H to 4L.
I don't know how many times I've seen people in off the road in the snow and the first thing they say is "I don't know what happened I was in 4 wheel drive". You were also doing 40 and tried to hit the brakes at the last second.
I have been driving 4X4's since 1957, I got rid of my last one three years ago. I have seen a ton of improvements in drive train, handling, rubber and power! But the fact remains the do not stop any faster of skid any less then they did before! Be careful experimenting with how fast they will go chit happens fast!
i would say if you are on dry or good traction surface than do not use 4 wheel drive. the front and rear gears are close in ratio but they are different and great traction will break the weakest link. if there is snow or sand or poor traction then 4 wheel would be fine. i say if you are above 20 MPH most of the run then 4 high if slow crawling then 4 low. we got our tree this week and there was mud everywhere i crawled around in 4 low then on the way out i hit 4 high and tossed some mud on the trails in the tree farm (we were the only people minus the workers and they said hit it).....mike
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