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4x4 Safety

What speed is safe to shift a 1995 GMC Seirra Z71 into 4 wheel high? And will driving freeway speeds in 4 wheel high in the snow hurt anything?

Mr. / Ms. Viking, Does Your Owner’s Manual Offer Any Help?

You can theoretically shift into 4hi whenever you want and go as fast as you want in it, but if conditions are slippery enough to allow/require you to use the four wheel drive you should NOT be driving anywhere close to typical freeway speeds. The 4wd doesn’t really help you brake and it is actually a downright hindrance to steering-- because the front and rear wheels are essentially locked together in 4wd, any kind of sudden turning maneuver even at moderate speeds will have unpredictable and dangerous results.

Four wheel drive is nice for getting you moving and keeping you going in bad conditions, but for crash avoidance it is no substitute for slowing down!

Owner’s manual? It will have the answers.

If conditions are bad enough to require 4WD, you should never be going anywhere near freeway speeds.

If this is the typical part-time 4WD system, it is suited ONLY to off-roading and to getting the vehicle out of a ditch. When the center differential is locked–as it is when a part-time 4WD system is activated–it can actually push the vehicle into a skid on a curve, due to its inability to produce the normal differential effect on that curve.

So, what the driver perceives as being more safe is actually less safe. This factor, coupled with excess speed, is undoubtedly why so many older Jeeps and the like wind up in a ditch, upside down, when driven on wintery road surfaces.

Even though it seems that most of the owners of these vehicles are not aware of it, they are making their vehicle less safe by driving at highway speeds in 4WD mode. The only alternative of course, is to operate the truck in its RWD mode, which is very poor on traction, but that is the reality with these vehicles.

If you want to be able to power all 4 wheels safely at more than slow driving speeds, you need a vehicle with AWD, not 4WD. And, if you want to be able to drive that AWD vehicle with maximum safety, use 4 winter tires, and SLOW DOWN.

No, but my question was answered on this sight.

Thank You

If this is the typical part-time 4WD system, it is suited ONLY to off-roading and to getting the vehicle out of a ditch.

UMM…WRONG…This system is GREAT for traveling down roads when it’s snowing. This system is one of the BEST systems to use in adverse (snow) weather. FAR FAR FAR superior to any fwd or and MOST awd systems. It drastically increases your driving traction.

When the center differential is locked–as it is when a part-time 4WD system is activated–it can actually push the vehicle into a skid on a curve, due to its inability to produce the normal differential effect on that curve.

I must assume that you’ve never driven a 4wd system in snow before. When we get weather like it has been the last few days (20" so far and falling)…the ONLY vehicles traveling around here are plows and 4wds.

They are not traveling at “freeway speeds”. The axle ratios of most 4WD TRUCKS have a slight off-set built in, say 4:11 in the rear and 4:10 in the front. This is GREAT for off-road conditions or 20" of snow as it allows the front end to “keep ahead” of the back end. But on pavement at highway speed, these vehicles do MUCH BETTER in 2-WD…

To answer the OP’s question, I would STOP before shifting into 4WD unless you want to risk a wreck and a $3000 gearbox replacement. SOME vehicles with a “live” front end can tolerate “shift on the fly” but if the front axle has some sort of free-wheeling hubs, and you ask a gear turning 3000 rpm to engage a gear connected to a driveshaft and axles turning zero RPM, something must give…

. But on pavement at highway speed, these vehicles do MUCH BETTER in 2-WD…

I’ll AGREE 100%…But the OP specifically said driving on snow. I was driving around all day today doing last minute shopping. Nice storm we’re having. The highways were plowed, but NOT dry. Still had a nice layer of snow and ice on the…IDEAL conditions to use 4wd. Just drudged along on I-93 doing 30…passed a couple cars doing 20.

Many, many later model transfer cases are designed to be shifted in to 4hi at speed.

This particular GMC has Insta-Trac which does allow you to shift at any speed.

Insta-Trac is a marketing term for a live front axle. These (and similar designs) can be shifted from 2H to 4H and back at any speed. Just let off the gas a little and snap it in…

I say 25 MPH. My owner’s manual may even agree with me but I can’t remember right now. One look at the transfer case will tell you to keep it in two wheel drive unless you are driving at low speeds and the roads are completely covered with snow and ice. If you get into a parking lot that is just wet; get it in two wheel drive before you have to turn twice. Of course, you have to read the manual.